class CRMDatabaseRouter:
    def db_for_read(self, model, **hints):
        if model._meta.app_label == 'seller_models':
            return 'seller_db'
        elif model._meta.app_label == 'customer_models':
            return 'customer_db'
        return 'default'

    def db_for_write(self, model, **hints):
        if model._meta.app_label == 'seller_models':
            return 'seller_db'
        elif model._meta.app_label == 'customer_models':
            return 'customer_db'
        return 'default'

    def allow_relation(self, obj1, obj2, **hints):
        return True

    def allow_migrate(self, db, app_label, model_name=None, **hints):
        if app_label == 'seller_models':
            return db == 'seller_db'
        elif app_label == 'customer_models':
            return db == 'customer_db'
        
        if db in ['seller_db', 'customer_db']:
            return False
        return True