How to mock .query.filter_by() in Flask-SqlAlchemy

Question

In brief

When testing a model class in Flask-SqlAlchemy, how can we mock the method .query.filter_by() so as to return the list of mocked model object

Answer 1

You'll have to mock the whole mapper class; accessing the query attribute on the mapper causes a session load:

@patch('app.model.some_model.SomeModel')
def test_some_case(self, some_model_mock):
    filter_by_mock = some_model_mock.query.filter_by
    # more test logic goes here

That's because the .query attribute is a descriptor object; accessing it triggers the binding to a session.

The alternative would be to mock out the _QueryProperty.__get__ method (which backs the .query attribute); only use this if you must test with actual SomeModel instances:

@patch('flask_sqlalchemy._QueryProperty.__get__')
def test_some_case(self, query_property_getter_mock):
    filter_by_mock = query_property_getter_mock.return_value.filter_by
    # more test logic goes here

Demo:

>>> from flask_sqlalchemy import SQLAlchemy
>>> db = SQLAlchemy()
>>> class SomeModel(db.Model):
...     id = db.Column(db.Integer, primary_key=True)
...
>>> from unittest import mock
>>> with mock.patch('__main__.SomeModel') as model_mock:
...     filter_by = model_mock.query.filter_by
...     SomeModel.query.filter_by(SomeModel.id == 'foo')
...

>>> with mock.patch('flask_sqlalchemy._QueryProperty.__get__') as query_property_getter_mock:
...     filter_by_mock = query_property_getter_mock.return_value.filter_by
...     SomeModel.query.filter_by(SomeModel.id == 'foo')
...