Fixed #23090: Document and enforce not double-squashing migrations

        # Load the operations from all those migrations and concat together

        operations = []

        for smigration in migrations_to_squash:

            if smigration.replaces:

                raise CommandError("You cannot squash squashed migrations! Please transition it to a normal migration first: https://docs.djangoproject.com/en/1.7/topics/migrations/#squashing-migrations")

            operations.extend(smigration.operations)

        if self.verbosity > 0:

way detailing the models and their relationships so we can improve the

optimizer to handle your case.

Once you've squashed your migration, you should then commit it alongside the

migrations it replaces and distribute this change to all running instances

of your application, making sure that they run ``migrate`` to store the change

in their database.

After this has been done, you must then transition the squashed migration to

a normal initial migration, by:

- Deleting all the migration files it replaces

- Removing the ``replaces`` argument in the ``Migration`` class of the

  squashed migration (this is how Django tells that it is a squashed migration)

.. note::

    Once you've squashed a migration, you should not then re-squash that squashed

    migration until you have fully transitioned it to a normal migration.

.. _migration-serializing:

Serializing values