Python - 模拟链式函数调用

我在单元测试下的一个方法中有以下声明.

db_employees = self.db._session.query(Employee).filter(Employee.dept ==   
    new_employee.dept).all()

我希望db_employees获得模拟员工列表.我尝试使用以下方法实现此目的

 m = MagickMock()
 m.return_value.filter().all().return_value = employees

员工是员工对象的列表.但这没效果.当我尝试打印任何属性的值时,它具有模拟值.这是代码的样子:

class Database(object):
    def __init__(self, user=None, passwd=None, db="sqlite:////tmp/emp.db"):
        try:
            engine = create_engine(db)
        except Exception:
            raise ValueError("Database '%s' does not exist." % db)

        def on_connect(conn, record):
            conn.execute('pragma foreign_keys=ON')

        if 'sqlite://' in db:
            event.listen(engine, 'connect', on_connect)
        Base.metadata.bind = engine
        DBSession = sessionmaker(bind=engine)
        self._session = DBSession()


class TestEmployee(MyEmployee):
    def setUp(self):
        self.db = emp.database.Database(db=options.connection)
        self.db._session._autoflush()

    @mock.patch.object(session.Session, 'add')     
    @mock.patch.object(session.Session, 'query')  
    def test_update(self, mock_query, mock_add):
        employees = [{'id': 1,
                      'name': 'Pradeep',
                      'department': 'IT',
                      'manager': 'John'}]
        mock_add.side_effect = self.add_side_effect
        mock_query.return_value = self.query_results()  
        self.update_employees(employees)

    def add_side_effect(self, instance, _warn=True):
        // Code to mock add
        // Values will be stored in a dict which will be used to 
        // check with expected value.

    def query_results(self):  
        m = MagicMock()  
        if self.count == 0:  
             m.return_value.filter.return_value.all.return_value = [employee]  
        elif:  
             m.return_value.filter.return_value.all.return_value = [department]  
        return m  

我有query_results作为测试调用查询两次的方法.首先是员工表,然后是部门表.

如何模拟此链接函数调用？

m = MagickMock()
m.session.query().filter().all.return_value = employees

https://docs.python.org/3/library/unittest.mock.html