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)
