# Django 2.2.9.dev20191202104323 文档

# django.db.migrations.operations.models 源代码

    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 ModelTuple, field_references_model

    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=None):
            return name.lower() == self.name_lower

        def reduce(self, operation, app_label=None):
            return (
                super().reduce(operation, app_label=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=None):
            name_lower = name.lower()
            if name_lower == self.name_lower:
                return True

            # Check we didn't inherit from the model
            model_tuple = ModelTuple(app_label, name_lower)
            for base in self.bases:
                if (base is not models.Model and isinstance(base, (models.base.ModelBase, str)) and
                        ModelTuple.from_model(base) == model_tuple):
                    return True

            # Check we have no FKs/M2Ms with it
            for _name, field in self.fields:
                if field_references_model(field, model_tuple):
                    return True
            return False

        def reduce(self, operation, app_label=None):
            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=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=None):
            # 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 = ModelTuple(app_label, self.old_name_lower)
            new_remote_model = '%s.%s' % (app_label, self.new_name)
            to_reload = []
            for (model_app_label, model_name), model_state in state.models.items():
                model_changed = False
                for index, (name, field) in enumerate(model_state.fields):
                    changed_field = None
                    remote_field = field.remote_field
                    if remote_field:
                        remote_model_tuple = ModelTuple.from_model(
                            remote_field.model, model_app_label, model_name
                        )
                        if remote_model_tuple == old_model_tuple:
                            changed_field = field.clone()
                            changed_field.remote_field.model = new_remote_model
                        through_model = getattr(remote_field, 'through', None)
                        if through_model:
                            through_model_tuple = ModelTuple.from_model(
                                through_model, model_app_label, model_name
                            )
                            if through_model_tuple == old_model_tuple:
                                if changed_field is None:
                                    changed_field = field.clone()
                                changed_field.remote_field.through = new_remote_model
                    if changed_field:
                        model_state.fields[index] = name, changed_field
                        model_changed = True
                if model_changed:
                    to_reload.append((model_app_label, model_name))
            # 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=None):
            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=None):
            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=app_label) or
                not operation.references_model(self.new_name, app_label)
            )

    class AlterModelTable(ModelOperation):
        """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)"
            )

        def reduce(self, operation, app_label=None):
            if isinstance(operation, (AlterModelTable, DeleteModel)) and self.name_lower == operation.name_lower:
                return [operation]
            return super().reduce(operation, app_label=app_label)

    class ModelOptionOperation(ModelOperation):
        def reduce(self, operation, app_label=None):
            if isinstance(operation, (self.__class__, DeleteModel)) and self.name_lower == operation.name_lower:
                return [operation]
            return super().reduce(operation, app_label=app_label)

    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=None):
            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=None):
            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)