More conversion to a ContextManager schema_editor

    # State-managing methods

    def start(self):

        """

        Marks the start of a schema-altering run.

        """

        self.deferred_sql = []

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

    def commit(self):

        """

        Finishes a schema-altering run.

        """

        for sql in self.deferred_sql:

            self.execute(sql)

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

    def rollback(self):

        """

        Tries to roll back a schema-altering run. Call instead of commit().

        """

        if not self.connection.features.can_rollback_ddl:

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

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

    def __enter__(self):

        self.start()

        self.deferred_sql = []

        atomic(self.connection.alias, self.connection.features.can_rollback_ddl).__enter__()

        return self

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

        if exc_type is None:

            self.commit()

            for sql in self.deferred_sql:

                self.execute(sql)

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

        else:

            self.rollback()

            # Continue propagating exception

            return None

    # Core utility functions

                model.render(self.app_cache)

        return self.app_cache

    @classmethod

    def from_app_cache(cls, app_cache):

        "Takes in an AppCache and returns a ProjectState matching it"

        for model in app_cache.get_models():

            print model

class ModelState(object):

    """