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Lib/site-packages/django/db/migrations/operations/__init__.py

@@ -0,0 +1,17 @@
+from .fields import AddField, AlterField, RemoveField, RenameField
+from .models import (
+    AddConstraint, AddIndex, AlterIndexTogether, AlterModelManagers,
+    AlterModelOptions, AlterModelTable, AlterOrderWithRespectTo,
+    AlterUniqueTogether, CreateModel, DeleteModel, RemoveConstraint,
+    RemoveIndex, RenameModel,
+)
+from .special import RunPython, RunSQL, SeparateDatabaseAndState
+
+__all__ = [
+    'CreateModel', 'DeleteModel', 'AlterModelTable', 'AlterUniqueTogether',
+    'RenameModel', 'AlterIndexTogether', 'AlterModelOptions', 'AddIndex',
+    'RemoveIndex', 'AddField', 'RemoveField', 'AlterField', 'RenameField',
+    'AddConstraint', 'RemoveConstraint',
+    'SeparateDatabaseAndState', 'RunSQL', 'RunPython',
+    'AlterOrderWithRespectTo', 'AlterModelManagers',
+]

Lib/site-packages/django/db/migrations/operations/base.py

@@ -0,0 +1,132 @@
+from django.db import router
+
+
+class Operation:
+    """
+    Base class for migration operations.
+
+    It's responsible for both mutating the in-memory model state
+    (see db/migrations/state.py) to represent what it performs, as well
+    as actually performing it against a live database.
+
+    Note that some operations won't modify memory state at all (e.g. data
+    copying operations), and some will need their modifications to be
+    optionally specified by the user (e.g. custom Python code snippets)
+
+    Due to the way this class deals with deconstruction, it should be
+    considered immutable.
+    """
+
+    # If this migration can be run in reverse.
+    # Some operations are impossible to reverse, like deleting data.
+    reversible = True
+
+    # Can this migration be represented as SQL? (things like RunPython cannot)
+    reduces_to_sql = True
+
+    # Should this operation be forced as atomic even on backends with no
+    # DDL transaction support (i.e., does it have no DDL, like RunPython)
+    atomic = False
+
+    # Should this operation be considered safe to elide and optimize across?
+    elidable = False
+
+    serialization_expand_args = []
+
+    def __new__(cls, *args, **kwargs):
+        # We capture the arguments to make returning them trivial
+        self = object.__new__(cls)
+        self._constructor_args = (args, kwargs)
+        return self
+
+    def deconstruct(self):
+        """
+        Return a 3-tuple of class import path (or just name if it lives
+        under django.db.migrations), positional arguments, and keyword
+        arguments.
+        """
+        return (
+            self.__class__.__name__,
+            self._constructor_args[0],
+            self._constructor_args[1],
+        )
+
+    def state_forwards(self, app_label, state):
+        """
+        Take the state from the previous migration, and mutate it
+        so that it matches what this migration would perform.
+        """
+        raise NotImplementedError('subclasses of Operation must provide a state_forwards() method')
+
+    def database_forwards(self, app_label, schema_editor, from_state, to_state):
+        """
+        Perform the mutation on the database schema in the normal
+        (forwards) direction.
+        """
+        raise NotImplementedError('subclasses of Operation must provide a database_forwards() method')
+
+    def database_backwards(self, app_label, schema_editor, from_state, to_state):
+        """
+        Perform the mutation on the database schema in the reverse
+        direction - e.g. if this were CreateModel, it would in fact
+        drop the model's table.
+        """
+        raise NotImplementedError('subclasses of Operation must provide a database_backwards() method')
+
+    def describe(self):
+        """
+        Output a brief summary of what the action does.
+        """
+        return "%s: %s" % (self.__class__.__name__, self._constructor_args)
+
+    def references_model(self, name, app_label):
+        """
+        Return True if there is a chance this operation references the given
+        model name (as a string), with an app label for accuracy.
+
+        Used for optimization. If in doubt, return True;
+        returning a false positive will merely make the optimizer a little
+        less efficient, while returning a false negative may result in an
+        unusable optimized migration.
+        """
+        return True
+
+    def references_field(self, model_name, name, app_label):
+        """
+        Return True if there is a chance this operation references the given
+        field name, with an app label for accuracy.
+
+        Used for optimization. If in doubt, return True.
+        """
+        return self.references_model(model_name, app_label)
+
+    def allow_migrate_model(self, connection_alias, model):
+        """
+        Return whether or not a model may be migrated.
+
+        This is a thin wrapper around router.allow_migrate_model() that
+        preemptively rejects any proxy, swapped out, or unmanaged model.
+        """
+        if not model._meta.can_migrate(connection_alias):
+            return False
+
+        return router.allow_migrate_model(connection_alias, model)
+
+    def reduce(self, operation, app_label):
+        """
+        Return either a list of operations the actual operation should be
+        replaced with or a boolean that indicates whether or not the specified
+        operation can be optimized across.
+        """
+        if self.elidable:
+            return [operation]
+        elif operation.elidable:
+            return [self]
+        return False
+
+    def __repr__(self):
+        return "<%s %s%s>" % (
+            self.__class__.__name__,
+            ", ".join(map(repr, self._constructor_args[0])),
+            ",".join(" %s=%r" % x for x in self._constructor_args[1].items()),
+        )

Lib/site-packages/django/db/migrations/operations/fields.py

@@ -0,0 +1,379 @@
+from django.core.exceptions import FieldDoesNotExist
+from django.db.models import NOT_PROVIDED
+from django.utils.functional import cached_property
+
+from .base import Operation
+from .utils import field_is_referenced, field_references, get_references
+
+
+class FieldOperation(Operation):
+    def __init__(self, model_name, name, field=None):
+        self.model_name = model_name
+        self.name = name
+        self.field = field
+
+    @cached_property
+    def model_name_lower(self):
+        return self.model_name.lower()
+
+    @cached_property
+    def name_lower(self):
+        return self.name.lower()
+
+    def is_same_model_operation(self, operation):
+        return self.model_name_lower == operation.model_name_lower
+
+    def is_same_field_operation(self, operation):
+        return self.is_same_model_operation(operation) and self.name_lower == operation.name_lower
+
+    def references_model(self, name, app_label):
+        name_lower = name.lower()
+        if name_lower == self.model_name_lower:
+            return True
+        if self.field:
+            return bool(field_references(
+                (app_label, self.model_name_lower), self.field, (app_label, name_lower)
+            ))
+        return False
+
+    def references_field(self, model_name, name, app_label):
+        model_name_lower = model_name.lower()
+        # Check if this operation locally references the field.
+        if model_name_lower == self.model_name_lower:
+            if name == self.name:
+                return True
+            elif self.field and hasattr(self.field, 'from_fields') and name in self.field.from_fields:
+                return True
+        # Check if this operation remotely references the field.
+        if self.field is None:
+            return False
+        return bool(field_references(
+            (app_label, self.model_name_lower),
+            self.field,
+            (app_label, model_name_lower),
+            name,
+        ))
+
+    def reduce(self, operation, app_label):
+        return (
+            super().reduce(operation, app_label) or
+            not operation.references_field(self.model_name, self.name, app_label)
+        )
+
+
+class AddField(FieldOperation):
+    """Add a field to a model."""
+
+    def __init__(self, model_name, name, field, preserve_default=True):
+        self.preserve_default = preserve_default
+        super().__init__(model_name, name, field)
+
+    def deconstruct(self):
+        kwargs = {
+            'model_name': self.model_name,
+            'name': self.name,
+            'field': self.field,
+        }
+        if self.preserve_default is not True:
+            kwargs['preserve_default'] = self.preserve_default
+        return (
+            self.__class__.__name__,
+            [],
+            kwargs
+        )
+
+    def state_forwards(self, app_label, state):
+        # If preserve default is off, don't use the default for future state
+        if not self.preserve_default:
+            field = self.field.clone()
+            field.default = NOT_PROVIDED
+        else:
+            field = self.field
+        state.models[app_label, self.model_name_lower].fields[self.name] = field
+        # Delay rendering of relationships if it's not a relational field
+        delay = not field.is_relation
+        state.reload_model(app_label, self.model_name_lower, delay=delay)
+
+    def database_forwards(self, app_label, schema_editor, from_state, to_state):
+        to_model = to_state.apps.get_model(app_label, self.model_name)
+        if self.allow_migrate_model(schema_editor.connection.alias, to_model):
+            from_model = from_state.apps.get_model(app_label, self.model_name)
+            field = to_model._meta.get_field(self.name)
+            if not self.preserve_default:
+                field.default = self.field.default
+            schema_editor.add_field(
+                from_model,
+                field,
+            )
+            if not self.preserve_default:
+                field.default = NOT_PROVIDED
+
+    def database_backwards(self, app_label, schema_editor, from_state, to_state):
+        from_model = from_state.apps.get_model(app_label, self.model_name)
+        if self.allow_migrate_model(schema_editor.connection.alias, from_model):
+            schema_editor.remove_field(from_model, from_model._meta.get_field(self.name))
+
+    def describe(self):
+        return "Add field %s to %s" % (self.name, self.model_name)
+
+    def reduce(self, operation, app_label):
+        if isinstance(operation, FieldOperation) and self.is_same_field_operation(operation):
+            if isinstance(operation, AlterField):
+                return [
+                    AddField(
+                        model_name=self.model_name,
+                        name=operation.name,
+                        field=operation.field,
+                    ),
+                ]
+            elif isinstance(operation, RemoveField):
+                return []
+            elif isinstance(operation, RenameField):
+                return [
+                    AddField(
+                        model_name=self.model_name,
+                        name=operation.new_name,
+                        field=self.field,
+                    ),
+                ]
+        return super().reduce(operation, app_label)
+
+
+class RemoveField(FieldOperation):
+    """Remove a field from a model."""
+
+    def deconstruct(self):
+        kwargs = {
+            'model_name': self.model_name,
+            'name': self.name,
+        }
+        return (
+            self.__class__.__name__,
+            [],
+            kwargs
+        )
+
+    def state_forwards(self, app_label, state):
+        model_state = state.models[app_label, self.model_name_lower]
+        old_field = model_state.fields.pop(self.name)
+        # Delay rendering of relationships if it's not a relational field
+        delay = not old_field.is_relation
+        state.reload_model(app_label, self.model_name_lower, delay=delay)
+
+    def database_forwards(self, app_label, schema_editor, from_state, to_state):
+        from_model = from_state.apps.get_model(app_label, self.model_name)
+        if self.allow_migrate_model(schema_editor.connection.alias, from_model):
+            schema_editor.remove_field(from_model, from_model._meta.get_field(self.name))
+
+    def database_backwards(self, app_label, schema_editor, from_state, to_state):
+        to_model = to_state.apps.get_model(app_label, self.model_name)
+        if self.allow_migrate_model(schema_editor.connection.alias, to_model):
+            from_model = from_state.apps.get_model(app_label, self.model_name)
+            schema_editor.add_field(from_model, to_model._meta.get_field(self.name))
+
+    def describe(self):
+        return "Remove field %s from %s" % (self.name, self.model_name)
+
+    def reduce(self, operation, app_label):
+        from .models import DeleteModel
+        if isinstance(operation, DeleteModel) and operation.name_lower == self.model_name_lower:
+            return [operation]
+        return super().reduce(operation, app_label)
+
+
+class AlterField(FieldOperation):
+    """
+    Alter a field's database column (e.g. null, max_length) to the provided
+    new field.
+    """
+
+    def __init__(self, model_name, name, field, preserve_default=True):
+        self.preserve_default = preserve_default
+        super().__init__(model_name, name, field)
+
+    def deconstruct(self):
+        kwargs = {
+            'model_name': self.model_name,
+            'name': self.name,
+            'field': self.field,
+        }
+        if self.preserve_default is not True:
+            kwargs['preserve_default'] = self.preserve_default
+        return (
+            self.__class__.__name__,
+            [],
+            kwargs
+        )
+
+    def state_forwards(self, app_label, state):
+        if not self.preserve_default:
+            field = self.field.clone()
+            field.default = NOT_PROVIDED
+        else:
+            field = self.field
+        model_state = state.models[app_label, self.model_name_lower]
+        model_state.fields[self.name] = field
+        # TODO: investigate if old relational fields must be reloaded or if it's
+        # sufficient if the new field is (#27737).
+        # Delay rendering of relationships if it's not a relational field and
+        # not referenced by a foreign key.
+        delay = (
+            not field.is_relation and
+            not field_is_referenced(
+                state, (app_label, self.model_name_lower), (self.name, field),
+            )
+        )
+        state.reload_model(app_label, self.model_name_lower, delay=delay)
+
+    def database_forwards(self, app_label, schema_editor, from_state, to_state):
+        to_model = to_state.apps.get_model(app_label, self.model_name)
+        if self.allow_migrate_model(schema_editor.connection.alias, to_model):
+            from_model = from_state.apps.get_model(app_label, self.model_name)
+            from_field = from_model._meta.get_field(self.name)
+            to_field = to_model._meta.get_field(self.name)
+            if not self.preserve_default:
+                to_field.default = self.field.default
+            schema_editor.alter_field(from_model, from_field, to_field)
+            if not self.preserve_default:
+                to_field.default = NOT_PROVIDED
+
+    def database_backwards(self, app_label, schema_editor, from_state, to_state):
+        self.database_forwards(app_label, schema_editor, from_state, to_state)
+
+    def describe(self):
+        return "Alter field %s on %s" % (self.name, self.model_name)
+
+    def reduce(self, operation, app_label):
+        if isinstance(operation, RemoveField) and self.is_same_field_operation(operation):
+            return [operation]
+        elif isinstance(operation, RenameField) and self.is_same_field_operation(operation):
+            return [
+                operation,
+                AlterField(
+                    model_name=self.model_name,
+                    name=operation.new_name,
+                    field=self.field,
+                ),
+            ]
+        return super().reduce(operation, app_label)
+
+
+class RenameField(FieldOperation):
+    """Rename a field on the model. Might affect db_column too."""
+
+    def __init__(self, model_name, old_name, new_name):
+        self.old_name = old_name
+        self.new_name = new_name
+        super().__init__(model_name, old_name)
+
+    @cached_property
+    def old_name_lower(self):
+        return self.old_name.lower()
+
+    @cached_property
+    def new_name_lower(self):
+        return self.new_name.lower()
+
+    def deconstruct(self):
+        kwargs = {
+            'model_name': self.model_name,
+            'old_name': self.old_name,
+            'new_name': self.new_name,
+        }
+        return (
+            self.__class__.__name__,
+            [],
+            kwargs
+        )
+
+    def state_forwards(self, app_label, state):
+        model_state = state.models[app_label, self.model_name_lower]
+        # Rename the field
+        fields = model_state.fields
+        try:
+            found = fields.pop(self.old_name)
+        except KeyError:
+            raise FieldDoesNotExist(
+                "%s.%s has no field named '%s'" % (app_label, self.model_name, self.old_name)
+            )
+        fields[self.new_name] = found
+        for field in fields.values():
+            # Fix from_fields to refer to the new field.
+            from_fields = getattr(field, 'from_fields', None)
+            if from_fields:
+                field.from_fields = tuple([
+                    self.new_name if from_field_name == self.old_name else from_field_name
+                    for from_field_name in from_fields
+                ])
+        # Fix index/unique_together to refer to the new field
+        options = model_state.options
+        for option in ('index_together', 'unique_together'):
+            if option in options:
+                options[option] = [
+                    [self.new_name if n == self.old_name else n for n in together]
+                    for together in options[option]
+                ]
+        # Fix to_fields to refer to the new field.
+        delay = True
+        references = get_references(
+            state, (app_label, self.model_name_lower), (self.old_name, found),
+        )
+        for *_, field, reference in references:
+            delay = False
+            if reference.to:
+                remote_field, to_fields = reference.to
+                if getattr(remote_field, 'field_name', None) == self.old_name:
+                    remote_field.field_name = self.new_name
+                if to_fields:
+                    field.to_fields = tuple([
+                        self.new_name if to_field_name == self.old_name else to_field_name
+                        for to_field_name in to_fields
+                    ])
+        state.reload_model(app_label, self.model_name_lower, delay=delay)
+
+    def database_forwards(self, app_label, schema_editor, from_state, to_state):
+        to_model = to_state.apps.get_model(app_label, self.model_name)
+        if self.allow_migrate_model(schema_editor.connection.alias, to_model):
+            from_model = from_state.apps.get_model(app_label, self.model_name)
+            schema_editor.alter_field(
+                from_model,
+                from_model._meta.get_field(self.old_name),
+                to_model._meta.get_field(self.new_name),
+            )
+
+    def database_backwards(self, app_label, schema_editor, from_state, to_state):
+        to_model = to_state.apps.get_model(app_label, self.model_name)
+        if self.allow_migrate_model(schema_editor.connection.alias, to_model):
+            from_model = from_state.apps.get_model(app_label, self.model_name)
+            schema_editor.alter_field(
+                from_model,
+                from_model._meta.get_field(self.new_name),
+                to_model._meta.get_field(self.old_name),
+            )
+
+    def describe(self):
+        return "Rename field %s on %s to %s" % (self.old_name, self.model_name, self.new_name)
+
+    def references_field(self, model_name, name, app_label):
+        return self.references_model(model_name, app_label) and (
+            name.lower() == self.old_name_lower or
+            name.lower() == self.new_name_lower
+        )
+
+    def reduce(self, operation, app_label):
+        if (isinstance(operation, RenameField) and
+                self.is_same_model_operation(operation) and
+                self.new_name_lower == operation.old_name_lower):
+            return [
+                RenameField(
+                    self.model_name,
+                    self.old_name,
+                    operation.new_name,
+                ),
+            ]
+        # Skip `FieldOperation.reduce` as we want to run `references_field`
+        # against self.new_name.
+        return (
+            super(FieldOperation, self).reduce(operation, app_label) or
+            not operation.references_field(self.model_name, self.new_name, app_label)
+        )

Lib/site-packages/django/db/migrations/operations/models.py

@@ -0,0 +1,859 @@
+from django.db import models
+from django.db.migrations.operations.base import Operation
+from django.db.migrations.state import ModelState
+from django.db.models.options import normalize_together
+from django.utils.functional import cached_property
+
+from .fields import (
+    AddField, AlterField, FieldOperation, RemoveField, RenameField,
+)
+from .utils import field_references, get_references, resolve_relation
+
+
+def _check_for_duplicates(arg_name, objs):
+    used_vals = set()
+    for val in objs:
+        if val in used_vals:
+            raise ValueError(
+                "Found duplicate value %s in CreateModel %s argument." % (val, arg_name)
+            )
+        used_vals.add(val)
+
+
+class ModelOperation(Operation):
+    def __init__(self, name):
+        self.name = name
+
+    @cached_property
+    def name_lower(self):
+        return self.name.lower()
+
+    def references_model(self, name, app_label):
+        return name.lower() == self.name_lower
+
+    def reduce(self, operation, app_label):
+        return (
+            super().reduce(operation, app_label) or
+            not operation.references_model(self.name, app_label)
+        )
+
+
+class CreateModel(ModelOperation):
+    """Create a model's table."""
+
+    serialization_expand_args = ['fields', 'options', 'managers']
+
+    def __init__(self, name, fields, options=None, bases=None, managers=None):
+        self.fields = fields
+        self.options = options or {}
+        self.bases = bases or (models.Model,)
+        self.managers = managers or []
+        super().__init__(name)
+        # Sanity-check that there are no duplicated field names, bases, or
+        # manager names
+        _check_for_duplicates('fields', (name for name, _ in self.fields))
+        _check_for_duplicates('bases', (
+            base._meta.label_lower if hasattr(base, '_meta') else
+            base.lower() if isinstance(base, str) else base
+            for base in self.bases
+        ))
+        _check_for_duplicates('managers', (name for name, _ in self.managers))
+
+    def deconstruct(self):
+        kwargs = {
+            'name': self.name,
+            'fields': self.fields,
+        }
+        if self.options:
+            kwargs['options'] = self.options
+        if self.bases and self.bases != (models.Model,):
+            kwargs['bases'] = self.bases
+        if self.managers and self.managers != [('objects', models.Manager())]:
+            kwargs['managers'] = self.managers
+        return (
+            self.__class__.__qualname__,
+            [],
+            kwargs
+        )
+
+    def state_forwards(self, app_label, state):
+        state.add_model(ModelState(
+            app_label,
+            self.name,
+            list(self.fields),
+            dict(self.options),
+            tuple(self.bases),
+            list(self.managers),
+        ))
+
+    def database_forwards(self, app_label, schema_editor, from_state, to_state):
+        model = to_state.apps.get_model(app_label, self.name)
+        if self.allow_migrate_model(schema_editor.connection.alias, model):
+            schema_editor.create_model(model)
+
+    def database_backwards(self, app_label, schema_editor, from_state, to_state):
+        model = from_state.apps.get_model(app_label, self.name)
+        if self.allow_migrate_model(schema_editor.connection.alias, model):
+            schema_editor.delete_model(model)
+
+    def describe(self):
+        return "Create %smodel %s" % ("proxy " if self.options.get("proxy", False) else "", self.name)
+
+    def references_model(self, name, app_label):
+        name_lower = name.lower()
+        if name_lower == self.name_lower:
+            return True
+
+        # Check we didn't inherit from the model
+        reference_model_tuple = (app_label, name_lower)
+        for base in self.bases:
+            if (base is not models.Model and isinstance(base, (models.base.ModelBase, str)) and
+                    resolve_relation(base, app_label) == reference_model_tuple):
+                return True
+
+        # Check we have no FKs/M2Ms with it
+        for _name, field in self.fields:
+            if field_references((app_label, self.name_lower), field, reference_model_tuple):
+                return True
+        return False
+
+    def reduce(self, operation, app_label):
+        if (isinstance(operation, DeleteModel) and
+                self.name_lower == operation.name_lower and
+                not self.options.get("proxy", False)):
+            return []
+        elif isinstance(operation, RenameModel) and self.name_lower == operation.old_name_lower:
+            return [
+                CreateModel(
+                    operation.new_name,
+                    fields=self.fields,
+                    options=self.options,
+                    bases=self.bases,
+                    managers=self.managers,
+                ),
+            ]
+        elif isinstance(operation, AlterModelOptions) and self.name_lower == operation.name_lower:
+            return [
+                CreateModel(
+                    self.name,
+                    fields=self.fields,
+                    options={**self.options, **operation.options},
+                    bases=self.bases,
+                    managers=self.managers,
+                ),
+            ]
+        elif isinstance(operation, AlterTogetherOptionOperation) and self.name_lower == operation.name_lower:
+            return [
+                CreateModel(
+                    self.name,
+                    fields=self.fields,
+                    options={**self.options, **{operation.option_name: operation.option_value}},
+                    bases=self.bases,
+                    managers=self.managers,
+                ),
+            ]
+        elif isinstance(operation, AlterOrderWithRespectTo) and self.name_lower == operation.name_lower:
+            return [
+                CreateModel(
+                    self.name,
+                    fields=self.fields,
+                    options={**self.options, 'order_with_respect_to': operation.order_with_respect_to},
+                    bases=self.bases,
+                    managers=self.managers,
+                ),
+            ]
+        elif isinstance(operation, FieldOperation) and self.name_lower == operation.model_name_lower:
+            if isinstance(operation, AddField):
+                return [
+                    CreateModel(
+                        self.name,
+                        fields=self.fields + [(operation.name, operation.field)],
+                        options=self.options,
+                        bases=self.bases,
+                        managers=self.managers,
+                    ),
+                ]
+            elif isinstance(operation, AlterField):
+                return [
+                    CreateModel(
+                        self.name,
+                        fields=[
+                            (n, operation.field if n == operation.name else v)
+                            for n, v in self.fields
+                        ],
+                        options=self.options,
+                        bases=self.bases,
+                        managers=self.managers,
+                    ),
+                ]
+            elif isinstance(operation, RemoveField):
+                options = self.options.copy()
+                for option_name in ('unique_together', 'index_together'):
+                    option = options.pop(option_name, None)
+                    if option:
+                        option = set(filter(bool, (
+                            tuple(f for f in fields if f != operation.name_lower) for fields in option
+                        )))
+                        if option:
+                            options[option_name] = option
+                order_with_respect_to = options.get('order_with_respect_to')
+                if order_with_respect_to == operation.name_lower:
+                    del options['order_with_respect_to']
+                return [
+                    CreateModel(
+                        self.name,
+                        fields=[
+                            (n, v)
+                            for n, v in self.fields
+                            if n.lower() != operation.name_lower
+                        ],
+                        options=options,
+                        bases=self.bases,
+                        managers=self.managers,
+                    ),
+                ]
+            elif isinstance(operation, RenameField):
+                options = self.options.copy()
+                for option_name in ('unique_together', 'index_together'):
+                    option = options.get(option_name)
+                    if option:
+                        options[option_name] = {
+                            tuple(operation.new_name if f == operation.old_name else f for f in fields)
+                            for fields in option
+                        }
+                order_with_respect_to = options.get('order_with_respect_to')
+                if order_with_respect_to == operation.old_name:
+                    options['order_with_respect_to'] = operation.new_name
+                return [
+                    CreateModel(
+                        self.name,
+                        fields=[
+                            (operation.new_name if n == operation.old_name else n, v)
+                            for n, v in self.fields
+                        ],
+                        options=options,
+                        bases=self.bases,
+                        managers=self.managers,
+                    ),
+                ]
+        return super().reduce(operation, app_label)
+
+
+class DeleteModel(ModelOperation):
+    """Drop a model's table."""
+
+    def deconstruct(self):
+        kwargs = {
+            'name': self.name,
+        }
+        return (
+            self.__class__.__qualname__,
+            [],
+            kwargs
+        )
+
+    def state_forwards(self, app_label, state):
+        state.remove_model(app_label, self.name_lower)
+
+    def database_forwards(self, app_label, schema_editor, from_state, to_state):
+        model = from_state.apps.get_model(app_label, self.name)
+        if self.allow_migrate_model(schema_editor.connection.alias, model):
+            schema_editor.delete_model(model)
+
+    def database_backwards(self, app_label, schema_editor, from_state, to_state):
+        model = to_state.apps.get_model(app_label, self.name)
+        if self.allow_migrate_model(schema_editor.connection.alias, model):
+            schema_editor.create_model(model)
+
+    def references_model(self, name, app_label):
+        # The deleted model could be referencing the specified model through
+        # related fields.
+        return True
+
+    def describe(self):
+        return "Delete model %s" % self.name
+
+
+class RenameModel(ModelOperation):
+    """Rename a model."""
+
+    def __init__(self, old_name, new_name):
+        self.old_name = old_name
+        self.new_name = new_name
+        super().__init__(old_name)
+
+    @cached_property
+    def old_name_lower(self):
+        return self.old_name.lower()
+
+    @cached_property
+    def new_name_lower(self):
+        return self.new_name.lower()
+
+    def deconstruct(self):
+        kwargs = {
+            'old_name': self.old_name,
+            'new_name': self.new_name,
+        }
+        return (
+            self.__class__.__qualname__,
+            [],
+            kwargs
+        )
+
+    def state_forwards(self, app_label, state):
+        # Add a new model.
+        renamed_model = state.models[app_label, self.old_name_lower].clone()
+        renamed_model.name = self.new_name
+        state.models[app_label, self.new_name_lower] = renamed_model
+        # Repoint all fields pointing to the old model to the new one.
+        old_model_tuple = (app_label, self.old_name_lower)
+        new_remote_model = '%s.%s' % (app_label, self.new_name)
+        to_reload = set()
+        for model_state, name, field, reference in get_references(state, old_model_tuple):
+            changed_field = None
+            if reference.to:
+                changed_field = field.clone()
+                changed_field.remote_field.model = new_remote_model
+            if reference.through:
+                if changed_field is None:
+                    changed_field = field.clone()
+                changed_field.remote_field.through = new_remote_model
+            if changed_field:
+                model_state.fields[name] = changed_field
+                to_reload.add((model_state.app_label, model_state.name_lower))
+        # Reload models related to old model before removing the old model.
+        state.reload_models(to_reload, delay=True)
+        # Remove the old model.
+        state.remove_model(app_label, self.old_name_lower)
+        state.reload_model(app_label, self.new_name_lower, delay=True)
+
+    def database_forwards(self, app_label, schema_editor, from_state, to_state):
+        new_model = to_state.apps.get_model(app_label, self.new_name)
+        if self.allow_migrate_model(schema_editor.connection.alias, new_model):
+            old_model = from_state.apps.get_model(app_label, self.old_name)
+            # Move the main table
+            schema_editor.alter_db_table(
+                new_model,
+                old_model._meta.db_table,
+                new_model._meta.db_table,
+            )
+            # Alter the fields pointing to us
+            for related_object in old_model._meta.related_objects:
+                if related_object.related_model == old_model:
+                    model = new_model
+                    related_key = (app_label, self.new_name_lower)
+                else:
+                    model = related_object.related_model
+                    related_key = (
+                        related_object.related_model._meta.app_label,
+                        related_object.related_model._meta.model_name,
+                    )
+                to_field = to_state.apps.get_model(
+                    *related_key
+                )._meta.get_field(related_object.field.name)
+                schema_editor.alter_field(
+                    model,
+                    related_object.field,
+                    to_field,
+                )
+            # Rename M2M fields whose name is based on this model's name.
+            fields = zip(old_model._meta.local_many_to_many, new_model._meta.local_many_to_many)
+            for (old_field, new_field) in fields:
+                # Skip self-referential fields as these are renamed above.
+                if new_field.model == new_field.related_model or not new_field.remote_field.through._meta.auto_created:
+                    continue
+                # Rename the M2M table that's based on this model's name.
+                old_m2m_model = old_field.remote_field.through
+                new_m2m_model = new_field.remote_field.through
+                schema_editor.alter_db_table(
+                    new_m2m_model,
+                    old_m2m_model._meta.db_table,
+                    new_m2m_model._meta.db_table,
+                )
+                # Rename the column in the M2M table that's based on this
+                # model's name.
+                schema_editor.alter_field(
+                    new_m2m_model,
+                    old_m2m_model._meta.get_field(old_model._meta.model_name),
+                    new_m2m_model._meta.get_field(new_model._meta.model_name),
+                )
+
+    def database_backwards(self, app_label, schema_editor, from_state, to_state):
+        self.new_name_lower, self.old_name_lower = self.old_name_lower, self.new_name_lower
+        self.new_name, self.old_name = self.old_name, self.new_name
+
+        self.database_forwards(app_label, schema_editor, from_state, to_state)
+
+        self.new_name_lower, self.old_name_lower = self.old_name_lower, self.new_name_lower
+        self.new_name, self.old_name = self.old_name, self.new_name
+
+    def references_model(self, name, app_label):
+        return (
+            name.lower() == self.old_name_lower or
+            name.lower() == self.new_name_lower
+        )
+
+    def describe(self):
+        return "Rename model %s to %s" % (self.old_name, self.new_name)
+
+    def reduce(self, operation, app_label):
+        if (isinstance(operation, RenameModel) and
+                self.new_name_lower == operation.old_name_lower):
+            return [
+                RenameModel(
+                    self.old_name,
+                    operation.new_name,
+                ),
+            ]
+        # Skip `ModelOperation.reduce` as we want to run `references_model`
+        # against self.new_name.
+        return (
+            super(ModelOperation, self).reduce(operation, app_label) or
+            not operation.references_model(self.new_name, app_label)
+        )
+
+
+class ModelOptionOperation(ModelOperation):
+    def reduce(self, operation, app_label):
+        if isinstance(operation, (self.__class__, DeleteModel)) and self.name_lower == operation.name_lower:
+            return [operation]
+        return super().reduce(operation, app_label)
+
+
+class AlterModelTable(ModelOptionOperation):
+    """Rename a model's table."""
+
+    def __init__(self, name, table):
+        self.table = table
+        super().__init__(name)
+
+    def deconstruct(self):
+        kwargs = {
+            'name': self.name,
+            'table': self.table,
+        }
+        return (
+            self.__class__.__qualname__,
+            [],
+            kwargs
+        )
+
+    def state_forwards(self, app_label, state):
+        state.models[app_label, self.name_lower].options["db_table"] = self.table
+        state.reload_model(app_label, self.name_lower, delay=True)
+
+    def database_forwards(self, app_label, schema_editor, from_state, to_state):
+        new_model = to_state.apps.get_model(app_label, self.name)
+        if self.allow_migrate_model(schema_editor.connection.alias, new_model):
+            old_model = from_state.apps.get_model(app_label, self.name)
+            schema_editor.alter_db_table(
+                new_model,
+                old_model._meta.db_table,
+                new_model._meta.db_table,
+            )
+            # Rename M2M fields whose name is based on this model's db_table
+            for (old_field, new_field) in zip(old_model._meta.local_many_to_many, new_model._meta.local_many_to_many):
+                if new_field.remote_field.through._meta.auto_created:
+                    schema_editor.alter_db_table(
+                        new_field.remote_field.through,
+                        old_field.remote_field.through._meta.db_table,
+                        new_field.remote_field.through._meta.db_table,
+                    )
+
+    def database_backwards(self, app_label, schema_editor, from_state, to_state):
+        return self.database_forwards(app_label, schema_editor, from_state, to_state)
+
+    def describe(self):
+        return "Rename table for %s to %s" % (
+            self.name,
+            self.table if self.table is not None else "(default)"
+        )
+
+
+class AlterTogetherOptionOperation(ModelOptionOperation):
+    option_name = None
+
+    def __init__(self, name, option_value):
+        if option_value:
+            option_value = set(normalize_together(option_value))
+        setattr(self, self.option_name, option_value)
+        super().__init__(name)
+
+    @cached_property
+    def option_value(self):
+        return getattr(self, self.option_name)
+
+    def deconstruct(self):
+        kwargs = {
+            'name': self.name,
+            self.option_name: self.option_value,
+        }
+        return (
+            self.__class__.__qualname__,
+            [],
+            kwargs
+        )
+
+    def state_forwards(self, app_label, state):
+        model_state = state.models[app_label, self.name_lower]
+        model_state.options[self.option_name] = self.option_value
+        state.reload_model(app_label, self.name_lower, delay=True)
+
+    def database_forwards(self, app_label, schema_editor, from_state, to_state):
+        new_model = to_state.apps.get_model(app_label, self.name)
+        if self.allow_migrate_model(schema_editor.connection.alias, new_model):
+            old_model = from_state.apps.get_model(app_label, self.name)
+            alter_together = getattr(schema_editor, 'alter_%s' % self.option_name)
+            alter_together(
+                new_model,
+                getattr(old_model._meta, self.option_name, set()),
+                getattr(new_model._meta, self.option_name, set()),
+            )
+
+    def database_backwards(self, app_label, schema_editor, from_state, to_state):
+        return self.database_forwards(app_label, schema_editor, from_state, to_state)
+
+    def references_field(self, model_name, name, app_label):
+        return (
+            self.references_model(model_name, app_label) and
+            (
+                not self.option_value or
+                any((name in fields) for fields in self.option_value)
+            )
+        )
+
+    def describe(self):
+        return "Alter %s for %s (%s constraint(s))" % (self.option_name, self.name, len(self.option_value or ''))
+
+
+class AlterUniqueTogether(AlterTogetherOptionOperation):
+    """
+    Change the value of unique_together to the target one.
+    Input value of unique_together must be a set of tuples.
+    """
+    option_name = 'unique_together'
+
+    def __init__(self, name, unique_together):
+        super().__init__(name, unique_together)
+
+
+class AlterIndexTogether(AlterTogetherOptionOperation):
+    """
+    Change the value of index_together to the target one.
+    Input value of index_together must be a set of tuples.
+    """
+    option_name = "index_together"
+
+    def __init__(self, name, index_together):
+        super().__init__(name, index_together)
+
+
+class AlterOrderWithRespectTo(ModelOptionOperation):
+    """Represent a change with the order_with_respect_to option."""
+
+    option_name = 'order_with_respect_to'
+
+    def __init__(self, name, order_with_respect_to):
+        self.order_with_respect_to = order_with_respect_to
+        super().__init__(name)
+
+    def deconstruct(self):
+        kwargs = {
+            'name': self.name,
+            'order_with_respect_to': self.order_with_respect_to,
+        }
+        return (
+            self.__class__.__qualname__,
+            [],
+            kwargs
+        )
+
+    def state_forwards(self, app_label, state):
+        model_state = state.models[app_label, self.name_lower]
+        model_state.options['order_with_respect_to'] = self.order_with_respect_to
+        state.reload_model(app_label, self.name_lower, delay=True)
+
+    def database_forwards(self, app_label, schema_editor, from_state, to_state):
+        to_model = to_state.apps.get_model(app_label, self.name)
+        if self.allow_migrate_model(schema_editor.connection.alias, to_model):
+            from_model = from_state.apps.get_model(app_label, self.name)
+            # Remove a field if we need to
+            if from_model._meta.order_with_respect_to and not to_model._meta.order_with_respect_to:
+                schema_editor.remove_field(from_model, from_model._meta.get_field("_order"))
+            # Add a field if we need to (altering the column is untouched as
+            # it's likely a rename)
+            elif to_model._meta.order_with_respect_to and not from_model._meta.order_with_respect_to:
+                field = to_model._meta.get_field("_order")
+                if not field.has_default():
+                    field.default = 0
+                schema_editor.add_field(
+                    from_model,
+                    field,
+                )
+
+    def database_backwards(self, app_label, schema_editor, from_state, to_state):
+        self.database_forwards(app_label, schema_editor, from_state, to_state)
+
+    def references_field(self, model_name, name, app_label):
+        return (
+            self.references_model(model_name, app_label) and
+            (
+                self.order_with_respect_to is None or
+                name == self.order_with_respect_to
+            )
+        )
+
+    def describe(self):
+        return "Set order_with_respect_to on %s to %s" % (self.name, self.order_with_respect_to)
+
+
+class AlterModelOptions(ModelOptionOperation):
+    """
+    Set new model options that don't directly affect the database schema
+    (like verbose_name, permissions, ordering). Python code in migrations
+    may still need them.
+    """
+
+    # Model options we want to compare and preserve in an AlterModelOptions op
+    ALTER_OPTION_KEYS = [
+        "base_manager_name",
+        "default_manager_name",
+        "default_related_name",
+        "get_latest_by",
+        "managed",
+        "ordering",
+        "permissions",
+        "default_permissions",
+        "select_on_save",
+        "verbose_name",
+        "verbose_name_plural",
+    ]
+
+    def __init__(self, name, options):
+        self.options = options
+        super().__init__(name)
+
+    def deconstruct(self):
+        kwargs = {
+            'name': self.name,
+            'options': self.options,
+        }
+        return (
+            self.__class__.__qualname__,
+            [],
+            kwargs
+        )
+
+    def state_forwards(self, app_label, state):
+        model_state = state.models[app_label, self.name_lower]
+        model_state.options = {**model_state.options, **self.options}
+        for key in self.ALTER_OPTION_KEYS:
+            if key not in self.options:
+                model_state.options.pop(key, False)
+        state.reload_model(app_label, self.name_lower, delay=True)
+
+    def database_forwards(self, app_label, schema_editor, from_state, to_state):
+        pass
+
+    def database_backwards(self, app_label, schema_editor, from_state, to_state):
+        pass
+
+    def describe(self):
+        return "Change Meta options on %s" % self.name
+
+
+class AlterModelManagers(ModelOptionOperation):
+    """Alter the model's managers."""
+
+    serialization_expand_args = ['managers']
+
+    def __init__(self, name, managers):
+        self.managers = managers
+        super().__init__(name)
+
+    def deconstruct(self):
+        return (
+            self.__class__.__qualname__,
+            [self.name, self.managers],
+            {}
+        )
+
+    def state_forwards(self, app_label, state):
+        model_state = state.models[app_label, self.name_lower]
+        model_state.managers = list(self.managers)
+        state.reload_model(app_label, self.name_lower, delay=True)
+
+    def database_forwards(self, app_label, schema_editor, from_state, to_state):
+        pass
+
+    def database_backwards(self, app_label, schema_editor, from_state, to_state):
+        pass
+
+    def describe(self):
+        return "Change managers on %s" % self.name
+
+
+class IndexOperation(Operation):
+    option_name = 'indexes'
+
+    @cached_property
+    def model_name_lower(self):
+        return self.model_name.lower()
+
+
+class AddIndex(IndexOperation):
+    """Add an index on a model."""
+
+    def __init__(self, model_name, index):
+        self.model_name = model_name
+        if not index.name:
+            raise ValueError(
+                "Indexes passed to AddIndex operations require a name "
+                "argument. %r doesn't have one." % index
+            )
+        self.index = index
+
+    def state_forwards(self, app_label, state):
+        model_state = state.models[app_label, self.model_name_lower]
+        model_state.options[self.option_name] = [*model_state.options[self.option_name], self.index.clone()]
+        state.reload_model(app_label, self.model_name_lower, delay=True)
+
+    def database_forwards(self, app_label, schema_editor, from_state, to_state):
+        model = to_state.apps.get_model(app_label, self.model_name)
+        if self.allow_migrate_model(schema_editor.connection.alias, model):
+            schema_editor.add_index(model, self.index)
+
+    def database_backwards(self, app_label, schema_editor, from_state, to_state):
+        model = from_state.apps.get_model(app_label, self.model_name)
+        if self.allow_migrate_model(schema_editor.connection.alias, model):
+            schema_editor.remove_index(model, self.index)
+
+    def deconstruct(self):
+        kwargs = {
+            'model_name': self.model_name,
+            'index': self.index,
+        }
+        return (
+            self.__class__.__qualname__,
+            [],
+            kwargs,
+        )
+
+    def describe(self):
+        return 'Create index %s on field(s) %s of model %s' % (
+            self.index.name,
+            ', '.join(self.index.fields),
+            self.model_name,
+        )
+
+
+class RemoveIndex(IndexOperation):
+    """Remove an index from a model."""
+
+    def __init__(self, model_name, name):
+        self.model_name = model_name
+        self.name = name
+
+    def state_forwards(self, app_label, state):
+        model_state = state.models[app_label, self.model_name_lower]
+        indexes = model_state.options[self.option_name]
+        model_state.options[self.option_name] = [idx for idx in indexes if idx.name != self.name]
+        state.reload_model(app_label, self.model_name_lower, delay=True)
+
+    def database_forwards(self, app_label, schema_editor, from_state, to_state):
+        model = from_state.apps.get_model(app_label, self.model_name)
+        if self.allow_migrate_model(schema_editor.connection.alias, model):
+            from_model_state = from_state.models[app_label, self.model_name_lower]
+            index = from_model_state.get_index_by_name(self.name)
+            schema_editor.remove_index(model, index)
+
+    def database_backwards(self, app_label, schema_editor, from_state, to_state):
+        model = to_state.apps.get_model(app_label, self.model_name)
+        if self.allow_migrate_model(schema_editor.connection.alias, model):
+            to_model_state = to_state.models[app_label, self.model_name_lower]
+            index = to_model_state.get_index_by_name(self.name)
+            schema_editor.add_index(model, index)
+
+    def deconstruct(self):
+        kwargs = {
+            'model_name': self.model_name,
+            'name': self.name,
+        }
+        return (
+            self.__class__.__qualname__,
+            [],
+            kwargs,
+        )
+
+    def describe(self):
+        return 'Remove index %s from %s' % (self.name, self.model_name)
+
+
+class AddConstraint(IndexOperation):
+    option_name = 'constraints'
+
+    def __init__(self, model_name, constraint):
+        self.model_name = model_name
+        self.constraint = constraint
+
+    def state_forwards(self, app_label, state):
+        model_state = state.models[app_label, self.model_name_lower]
+        model_state.options[self.option_name] = [*model_state.options[self.option_name], self.constraint]
+        state.reload_model(app_label, self.model_name_lower, delay=True)
+
+    def database_forwards(self, app_label, schema_editor, from_state, to_state):
+        model = to_state.apps.get_model(app_label, self.model_name)
+        if self.allow_migrate_model(schema_editor.connection.alias, model):
+            schema_editor.add_constraint(model, self.constraint)
+
+    def database_backwards(self, app_label, schema_editor, from_state, to_state):
+        model = to_state.apps.get_model(app_label, self.model_name)
+        if self.allow_migrate_model(schema_editor.connection.alias, model):
+            schema_editor.remove_constraint(model, self.constraint)
+
+    def deconstruct(self):
+        return self.__class__.__name__, [], {
+            'model_name': self.model_name,
+            'constraint': self.constraint,
+        }
+
+    def describe(self):
+        return 'Create constraint %s on model %s' % (self.constraint.name, self.model_name)
+
+
+class RemoveConstraint(IndexOperation):
+    option_name = 'constraints'
+
+    def __init__(self, model_name, name):
+        self.model_name = model_name
+        self.name = name
+
+    def state_forwards(self, app_label, state):
+        model_state = state.models[app_label, self.model_name_lower]
+        constraints = model_state.options[self.option_name]
+        model_state.options[self.option_name] = [c for c in constraints if c.name != self.name]
+        state.reload_model(app_label, self.model_name_lower, delay=True)
+
+    def database_forwards(self, app_label, schema_editor, from_state, to_state):
+        model = to_state.apps.get_model(app_label, self.model_name)
+        if self.allow_migrate_model(schema_editor.connection.alias, model):
+            from_model_state = from_state.models[app_label, self.model_name_lower]
+            constraint = from_model_state.get_constraint_by_name(self.name)
+            schema_editor.remove_constraint(model, constraint)
+
+    def database_backwards(self, app_label, schema_editor, from_state, to_state):
+        model = to_state.apps.get_model(app_label, self.model_name)
+        if self.allow_migrate_model(schema_editor.connection.alias, model):
+            to_model_state = to_state.models[app_label, self.model_name_lower]
+            constraint = to_model_state.get_constraint_by_name(self.name)
+            schema_editor.add_constraint(model, constraint)
+
+    def deconstruct(self):
+        return self.__class__.__name__, [], {
+            'model_name': self.model_name,
+            'name': self.name,
+        }
+
+    def describe(self):
+        return 'Remove constraint %s from model %s' % (self.name, self.model_name)

Lib/site-packages/django/db/migrations/operations/special.py

@@ -0,0 +1,203 @@
+from django.db import router
+
+from .base import Operation
+
+
+class SeparateDatabaseAndState(Operation):
+    """
+    Take two lists of operations - ones that will be used for the database,
+    and ones that will be used for the state change. This allows operations
+    that don't support state change to have it applied, or have operations
+    that affect the state or not the database, or so on.
+    """
+
+    serialization_expand_args = ['database_operations', 'state_operations']
+
+    def __init__(self, database_operations=None, state_operations=None):
+        self.database_operations = database_operations or []
+        self.state_operations = state_operations or []
+
+    def deconstruct(self):
+        kwargs = {}
+        if self.database_operations:
+            kwargs['database_operations'] = self.database_operations
+        if self.state_operations:
+            kwargs['state_operations'] = self.state_operations
+        return (
+            self.__class__.__qualname__,
+            [],
+            kwargs
+        )
+
+    def state_forwards(self, app_label, state):
+        for state_operation in self.state_operations:
+            state_operation.state_forwards(app_label, state)
+
+    def database_forwards(self, app_label, schema_editor, from_state, to_state):
+        # We calculate state separately in here since our state functions aren't useful
+        for database_operation in self.database_operations:
+            to_state = from_state.clone()
+            database_operation.state_forwards(app_label, to_state)
+            database_operation.database_forwards(app_label, schema_editor, from_state, to_state)
+            from_state = to_state
+
+    def database_backwards(self, app_label, schema_editor, from_state, to_state):
+        # We calculate state separately in here since our state functions aren't useful
+        to_states = {}
+        for dbop in self.database_operations:
+            to_states[dbop] = to_state
+            to_state = to_state.clone()
+            dbop.state_forwards(app_label, to_state)
+        # to_state now has the states of all the database_operations applied
+        # which is the from_state for the backwards migration of the last
+        # operation.
+        for database_operation in reversed(self.database_operations):
+            from_state = to_state
+            to_state = to_states[database_operation]
+            database_operation.database_backwards(app_label, schema_editor, from_state, to_state)
+
+    def describe(self):
+        return "Custom state/database change combination"
+
+
+class RunSQL(Operation):
+    """
+    Run some raw SQL. A reverse SQL statement may be provided.
+
+    Also accept a list of operations that represent the state change effected
+    by this SQL change, in case it's custom column/table creation/deletion.
+    """
+    noop = ''
+
+    def __init__(self, sql, reverse_sql=None, state_operations=None, hints=None, elidable=False):
+        self.sql = sql
+        self.reverse_sql = reverse_sql
+        self.state_operations = state_operations or []
+        self.hints = hints or {}
+        self.elidable = elidable
+
+    def deconstruct(self):
+        kwargs = {
+            'sql': self.sql,
+        }
+        if self.reverse_sql is not None:
+            kwargs['reverse_sql'] = self.reverse_sql
+        if self.state_operations:
+            kwargs['state_operations'] = self.state_operations
+        if self.hints:
+            kwargs['hints'] = self.hints
+        return (
+            self.__class__.__qualname__,
+            [],
+            kwargs
+        )
+
+    @property
+    def reversible(self):
+        return self.reverse_sql is not None
+
+    def state_forwards(self, app_label, state):
+        for state_operation in self.state_operations:
+            state_operation.state_forwards(app_label, state)
+
+    def database_forwards(self, app_label, schema_editor, from_state, to_state):
+        if router.allow_migrate(schema_editor.connection.alias, app_label, **self.hints):
+            self._run_sql(schema_editor, self.sql)
+
+    def database_backwards(self, app_label, schema_editor, from_state, to_state):
+        if self.reverse_sql is None:
+            raise NotImplementedError("You cannot reverse this operation")
+        if router.allow_migrate(schema_editor.connection.alias, app_label, **self.hints):
+            self._run_sql(schema_editor, self.reverse_sql)
+
+    def describe(self):
+        return "Raw SQL operation"
+
+    def _run_sql(self, schema_editor, sqls):
+        if isinstance(sqls, (list, tuple)):
+            for sql in sqls:
+                params = None
+                if isinstance(sql, (list, tuple)):
+                    elements = len(sql)
+                    if elements == 2:
+                        sql, params = sql
+                    else:
+                        raise ValueError("Expected a 2-tuple but got %d" % elements)
+                schema_editor.execute(sql, params=params)
+        elif sqls != RunSQL.noop:
+            statements = schema_editor.connection.ops.prepare_sql_script(sqls)
+            for statement in statements:
+                schema_editor.execute(statement, params=None)
+
+
+class RunPython(Operation):
+    """
+    Run Python code in a context suitable for doing versioned ORM operations.
+    """
+
+    reduces_to_sql = False
+
+    def __init__(self, code, reverse_code=None, atomic=None, hints=None, elidable=False):
+        self.atomic = atomic
+        # Forwards code
+        if not callable(code):
+            raise ValueError("RunPython must be supplied with a callable")
+        self.code = code
+        # Reverse code
+        if reverse_code is None:
+            self.reverse_code = None
+        else:
+            if not callable(reverse_code):
+                raise ValueError("RunPython must be supplied with callable arguments")
+            self.reverse_code = reverse_code
+        self.hints = hints or {}
+        self.elidable = elidable
+
+    def deconstruct(self):
+        kwargs = {
+            'code': self.code,
+        }
+        if self.reverse_code is not None:
+            kwargs['reverse_code'] = self.reverse_code
+        if self.atomic is not None:
+            kwargs['atomic'] = self.atomic
+        if self.hints:
+            kwargs['hints'] = self.hints
+        return (
+            self.__class__.__qualname__,
+            [],
+            kwargs
+        )
+
+    @property
+    def reversible(self):
+        return self.reverse_code is not None
+
+    def state_forwards(self, app_label, state):
+        # RunPython objects have no state effect. To add some, combine this
+        # with SeparateDatabaseAndState.
+        pass
+
+    def database_forwards(self, app_label, schema_editor, from_state, to_state):
+        # RunPython has access to all models. Ensure that all models are
+        # reloaded in case any are delayed.
+        from_state.clear_delayed_apps_cache()
+        if router.allow_migrate(schema_editor.connection.alias, app_label, **self.hints):
+            # We now execute the Python code in a context that contains a 'models'
+            # object, representing the versioned models as an app registry.
+            # We could try to override the global cache, but then people will still
+            # use direct imports, so we go with a documentation approach instead.
+            self.code(from_state.apps, schema_editor)
+
+    def database_backwards(self, app_label, schema_editor, from_state, to_state):
+        if self.reverse_code is None:
+            raise NotImplementedError("You cannot reverse this operation")
+        if router.allow_migrate(schema_editor.connection.alias, app_label, **self.hints):
+            self.reverse_code(from_state.apps, schema_editor)
+
+    def describe(self):
+        return "Raw Python operation"
+
+    @staticmethod
+    def noop(apps, schema_editor):
+        return None

Lib/site-packages/django/db/migrations/operations/utils.py

@@ -0,0 +1,101 @@
+from collections import namedtuple
+
+from django.db.models.fields.related import RECURSIVE_RELATIONSHIP_CONSTANT
+
+
+def resolve_relation(model, app_label=None, model_name=None):
+    """
+    Turn a model class or model reference string and return a model tuple.
+
+    app_label and model_name are used to resolve the scope of recursive and
+    unscoped model relationship.
+    """
+    if isinstance(model, str):
+        if model == RECURSIVE_RELATIONSHIP_CONSTANT:
+            if app_label is None or model_name is None:
+                raise TypeError(
+                    'app_label and model_name must be provided to resolve '
+                    'recursive relationships.'
+                )
+            return app_label, model_name
+        if '.' in model:
+            return tuple(model.lower().split('.', 1))
+        if app_label is None:
+            raise TypeError(
+                'app_label must be provided to resolve unscoped model '
+                'relationships.'
+            )
+        return app_label, model.lower()
+    return model._meta.app_label, model._meta.model_name
+
+
+FieldReference = namedtuple('FieldReference', 'to through')
+
+
+def field_references(
+    model_tuple,
+    field,
+    reference_model_tuple,
+    reference_field_name=None,
+    reference_field=None,
+):
+    """
+    Return either False or a FieldReference if `field` references provided
+    context.
+
+    False positives can be returned if `reference_field_name` is provided
+    without `reference_field` because of the introspection limitation it
+    incurs. This should not be an issue when this function is used to determine
+    whether or not an optimization can take place.
+    """
+    remote_field = field.remote_field
+    if not remote_field:
+        return False
+    references_to = None
+    references_through = None
+    if resolve_relation(remote_field.model, *model_tuple) == reference_model_tuple:
+        to_fields = getattr(field, 'to_fields', None)
+        if (
+            reference_field_name is None or
+            # Unspecified to_field(s).
+            to_fields is None or
+            # Reference to primary key.
+            (None in to_fields and (reference_field is None or reference_field.primary_key)) or
+            # Reference to field.
+            reference_field_name in to_fields
+        ):
+            references_to = (remote_field, to_fields)
+    through = getattr(remote_field, 'through', None)
+    if through and resolve_relation(through, *model_tuple) == reference_model_tuple:
+        through_fields = remote_field.through_fields
+        if (
+            reference_field_name is None or
+            # Unspecified through_fields.
+            through_fields is None or
+            # Reference to field.
+            reference_field_name in through_fields
+        ):
+            references_through = (remote_field, through_fields)
+    if not (references_to or references_through):
+        return False
+    return FieldReference(references_to, references_through)
+
+
+def get_references(state, model_tuple, field_tuple=()):
+    """
+    Generator of (model_state, name, field, reference) referencing
+    provided context.
+
+    If field_tuple is provided only references to this particular field of
+    model_tuple will be generated.
+    """
+    for state_model_tuple, model_state in state.models.items():
+        for name, field in model_state.fields.items():
+            reference = field_references(state_model_tuple, field, model_tuple, *field_tuple)
+            if reference:
+                yield model_state, name, field, reference
+
+
+def field_is_referenced(state, model_tuple, field_tuple):
+    """Return whether `field_tuple` is referenced by any state models."""
+    return next(get_references(state, model_tuple, field_tuple), None) is not None