Method _migrate_all_backwards

django/db/migrations/executor.py:176–239

Take a list of 2-tuples of the form (migration instance, True) and unapply them in reverse order they occur in the full_plan. Since unapplying a migration requires the project state prior to that migration, Django will compute the migration states before each of the

(self, plan, full_plan, fake)

        return state

    def _migrate_all_backwards(self, plan, full_plan, fake):
        """
        Take a list of 2-tuples of the form (migration instance, True) and
        unapply them in reverse order they occur in the full_plan.

        Since unapplying a migration requires the project state prior to that
        migration, Django will compute the migration states before each of them
        in a first run over the plan and then unapply them in a second run over
        the plan.
        """
        migrations_to_run = {m[0] for m in plan}
        # Holds all migration states prior to the migrations being unapplied
        states = {}
        state = self._create_project_state()
        applied_migrations = {
            self.loader.graph.nodes[key]
            for key in self.loader.applied_migrations
            if key in self.loader.graph.nodes
        }
        if self.progress_callback:
            self.progress_callback("render_start")
        for migration, _ in full_plan:
            if not migrations_to_run:
                # We remove every migration that we applied from this set so
                # that we can bail out once the last migration has been applied
                # and don't always run until the very end of the migration
                # process.
                break
            if migration in migrations_to_run:
                if "apps" not in state.__dict__:
                    state.apps  # Render all -- performance critical
                # The state before this migration
                states[migration] = state
                # The old state keeps as-is, we continue with the new state
                state = migration.mutate_state(state, preserve=True)
                migrations_to_run.remove(migration)
            elif migration in applied_migrations:
                # Only mutate the state if the migration is actually applied
                # to make sure the resulting state doesn't include changes
                # from unrelated migrations.
                migration.mutate_state(state, preserve=False)
        if self.progress_callback:
            self.progress_callback("render_success")

        for migration, _ in plan:
            self.unapply_migration(states[migration], migration, fake=fake)
            applied_migrations.remove(migration)

        # Generate the post migration state by starting from the state before
        # the last migration is unapplied and mutating it to include all the
        # remaining applied migrations.
        last_unapplied_migration = plan[-1][0]
        state = states[last_unapplied_migration]
        # Avoid mutating state with apps rendered as it's an expensive
        # operation.
        del state.apps
        for index, (migration, _) in enumerate(full_plan):
            if migration == last_unapplied_migration: