class MultiDbRouter(object):
    """
    A router to control all database operations on models in the
    auth application.
    """

    def db_for_read(self, model, **hints):
        """
        Attempts to read auth models go to auth_db.
        """
        if hasattr(model._meta, 'in_db'):
            return model._meta.in_db
        return None

    def db_for_write(self, model, **hints):
        """
        Attempts to write auth models go to auth_db.
        """
        if hasattr(model._meta, 'in_db'):
            return model._meta.in_db
        return None

    def allow_migrate(self, db, app_label, model=None, **hints):
        """
        Make sure the auth app only appears in the 'auth_db'
        database.
        """
        if model and model._meta:
            if hasattr(model._meta, 'in_db'):
                return model._meta.in_db == db
        return None