Problems trying to mock a Model within Flask-SQLAlchemy

Question

I\'m testing a Flask application that have some SQLAlchemy models using Flask-SQLAlchemy and I\'m having some problems trying to mock a few models to some methods that receive s

Answer 1

I found another way around this problem. The basic idea is to control the access to static attributes. I used pytest and mocker, but the code could be adapted to use unittest.

Let's look at a working code example and than explain it:

import pytest

import datetime

import database

from actions import get_user_age


@pytest.fixture
def mock_user_class(mocker):
    class MockedUserMeta(type):
        static_instance = mocker.MagicMock(spec=database.User)

        def __getattr__(cls, key):
            return MockedUserMeta.static_instance.__getattr__(key)

    class MockedUser(metaclass=MockedUserMeta):
        original_cls = database.User
        instances = []

        def __new__(cls, *args, **kwargs):
            MockedUser.instances.append(
                mocker.MagicMock(spec=MockedUser.original_cls))
            MockedUser.instances[-1].__class__ = MockedUser
            return MockedUser.instances[-1]

    mocker.patch('database.User', new=MockedUser)


class TestModels:
    def test_test_get_user_age(self, mock_user_class):
        user = database.User()
        user.birthday = datetime.date(year=1987, month=12, day=1)
        print(get_user_age(user))

The test is pretty clear and to the point. The fixture does all the heavy lifting:

• MockedUser would replace the original User class - it would create a new mock object with the right spec every time it's needed
• The purpose of MockedUserMeta has to be explained a bit further: SQLAlchemy has a nasty syntax which involves static functions. Imagine your tested code has a line similar to this from_db = User.query.filter(User.id == 20).one(), you should have a way to mock the response: MockedUserMeta.static_instance.query.filter.return_value.one.return_value.username = 'mocked_username'

This is the best method that I found which allows to have tests without any db access and without any flask app, while allowing to mock SQLAlchemy query results.

Since I don't like writing this boilerplate over and over, I have created a helper library to do it for me. Here is the code I wrote to generate the needed stuff for your example:

from mock_autogen.pytest_mocker import PytestMocker
print(PytestMocker(database).mock_classes().mock_classes_static().generate())

The output is:

class MockedUserMeta(type):
    static_instance = mocker.MagicMock(spec=database.User)

    def __getattr__(cls, key):
        return MockedUserMeta.static_instance.__getattr__(key)

class MockedUser(metaclass=MockedUserMeta):
    original_cls = database.User
    instances = []

    def __new__(cls, *args, **kwargs):
        MockedUser.instances.append(mocker.MagicMock(spec=MockedUser.original_cls))
        MockedUser.instances[-1].__class__ = MockedUser
        return MockedUser.instances[-1]

mocker.patch('database.User', new=MockedUser)

Which is exactly what I needed to place in my fixture.