Python - Mocking chained function calls

Question

I have the following statement in one of the methods under unit test.

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

I want db_employees to get mock list of employees. I tried to achieve this using:

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

where employees is a list of employee object. But this did not work. When I try to print the value of any attribute, it has a mock value. This is how the code looks:

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   

I have query_results as the method under test calls query twice. First the employee table and next the department table.

How do I mock this chained function call?

Answer 1

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

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

Answer 2

I found a solution to a similar problem where I needed to mock out a nested set of filtering calls.

Given code under test similar to the following:

interesting_cats = (session.query(Cats)                            .filter(Cat.fur_type == 'furry')                            .filter(Cat.voice == 'meowrific')                            .filter(Cat.color == 'orande')                            .all()) 

You can setup mocks like the following:

mock_session_response = MagicMock() # This is the magic - create a mock loop mock_session_response.filter.return_value = mock_session_response # We can exit the loop with a call to 'all' mock_session_response.all.return_value = provided_cats  mock_session = MagicMock(spec=Session) mock_session.query.return_value = mock_session_response