Turn SchemaEditor into a context manager

import sys

import hashlib

from django.db.backends.creation import BaseDatabaseCreation

from django.db.backends.util import truncate_name

from django.utils.log import getLogger

from django.db.models.fields.related import ManyToManyField

from django.db.transaction import atomic

logger = getLogger('django.db.backends.schema')

        Marks the start of a schema-altering run.

        """

        self.deferred_sql = []

        self.old_autocommit = self.connection.autocommit

        if self.connection.autocommit:

            self.connection.set_autocommit(False)

        atomic(self.connection.alias).__enter__()

    def commit(self):

        """

        """

        for sql in self.deferred_sql:

            self.execute(sql)

        self.connection.commit()

        self.connection.set_autocommit(self.old_autocommit)

        atomic(self.connection.alias).__exit__(None, None, None)

    def rollback(self):

        """

        """

        if not self.connection.features.can_rollback_ddl:

            raise RuntimeError("Cannot rollback schema changes on this backend")

        self.connection.rollback()

        self.connection.set_autocommit(self.old_autocommit)

        atomic(self.connection.alias).__exit__(*sys.exc_info())

    def __enter__(self):

        self.start()

        return self

    def __exit__(self, exc_type, exc_value, traceback):

        if exc_type is None:

            self.commit()

        else:

            self.rollback()

    # Core utility functions

        if self.Migration._meta.db_table in self.connection.introspection.get_table_list(self.connection.cursor()):

            return

        # Make the table

        editor = self.connection.schema_editor()

        editor.start()

        with self.connection.schema_editor() as editor:

            editor.create_model(self.Migration)

        editor.commit()

    def applied_migrations(self):

        """