Fixed #8569 -- Ensure that apps are correctly set up before introspecting them

    classmethod in the end if it is provided in cls. The signature of the

    custom validation classmethod should be: def validate(cls, model).

    """

    # Before we can introspect models, they need to be fully loaded so that

    # inter-relations are set up correctly. We force that here.

    models.get_apps()

    opts = model._meta

    validate_base(cls, model)