Using python mocking library on sqlalchemy

Question

I'm using sqlalchemy to query my databases for a project. Side-by-side, I'm new to unit testing and I'm trying to learn how can I make unit tests to test my database. I tried to use mocking library to test but so far, it seems to be very difficult.

So I made a piece of code which creates a Session object. This object is used to connect to database.

from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.exc import OperationalError, ArgumentError

test_db_string = 'postgresql+psycopg2://testdb:hello@localhost/' \
                   'test_databasetable'
def get_session(database_connection_string):
    try:
        Base = declarative_base()
        engine = create_engine(database_connection_string)
        Base.metadata.bind = engine
        DBSession = sessionmaker(bind=engine)
        session = DBSession()
        connection = session.connection()
        return session
    except OperationalError:
        return None
    except ArgumentError:
        return None

So I made a unit test case for this function:

import mock
import unittest
from mock import sentinel
import get_session

class TestUtilMock(unittest.TestCase):

    @mock.patch('app.Utilities.util.create_engine')  # mention the whole path
    @mock.patch('app.Utilities.util.sessionmaker')
    @mock.patch('app.Utilities.util.declarative_base')
    def test_get_session1(self, mock_delarative_base, mock_sessionmaker,
                         mock_create_engine):
        mock_create_engine.return_value = sentinel.engine
        get_session('any_path')
        mock_delarative_base.called
        mock_create_engine.assert_called_once_with('any_path')
        mock_sessionmaker.assert_called_once_with(bind=sentinel.engine)

As you see in my unit test, I cannot test code in get_session() starting from line session = DBSession(). So far after googling, I cannot find out if mock value returned can be also used to mock function calls - something like, I mock session object and verify if I called session.connection()

Is the above method of writing unit test case the right way? Is there a better method to do this?

Answer 1

First of all you can test session = DBSession() line also by

self.assertEqual(get_session('any_path'), mock_sessionmaker.return_value.return_value)

Moreover, mock_delarative_base.called is not an assert and cannot fail. Replace it with

self.assertTrue(mock_delarative_base.called)

General considerations

Writing test like these can be very time consuming and make your production code really coupled to test code. It is better to write your own wrapper that presents a comfortable interface for your business code and test it with some mocks and real (simple) tests: you should trust sqlalchemy implementation and just test how your wrapper calls it

After you can either mock your wrapper by a fake object that you can control or use mocks and check how your code calls it but your wrapper will present just some business interface and mocking it should be very simple.