Mocking a Django Queryset in order to test a function that takes a queryset
Question:
I have a utility function in my Django project, it takes a queryset, gets some data from it and returns a result. I’d like to write some tests for this function. Is there anyway to ‘mock’ a QuerySet? I’d like to create an object that doesn’t touch the database, and i can provide it with a list of values to use (i.e. some fake rows) and then it’ll act just like a queryset, and will allow someone to do field lookups on it/filter/get/all etc.
Does anything like this exist already?
Answers:
Not that I know of, but why not use an actual queryset? The test framework is all set up to allow you to create sample data within your test, and the database is re-created on every test, so there doesn’t seem to be any reason not to use the real thing.
Of course you can mock a QuerySet, you can mock anything.
You can create an object yourself, and give it the interface you need, and have it return any data you like. At heart, mocking is nothing more than providing a “test double” that acts enough like the real thing for your tests’ purposes.
The low-tech way to get started is to define an object:
class MockQuerySet(object):
pass
then create one of these, and hand it to your test. The test will fail, likely on an AttributeError. That will tell you what you need to implement on your MockQuerySet. Repeat until your object is rich enough for your tests.
One first advice would be to split the function in two parts, one that creates the queryset
and one that manipulates the output of it. In this way testing the second part is straightforward.
For the database problem, I investigated if django uses sqlite-in-memory and I found out that
recent version of django uses the sqlite -in-memory database, from The django unittest page:
When using the SQLite database engine the tests will by default use an
in-memory database (i.e., the database will be created in memory,
bypassing the filesystem entirely!).
Mocking the QuerySet object will not make you exercise its full logic.
I am having the same issue, and it looks like some nice person has written a library for mocking QuerySets, it is called mock-django and the specific code you will need is here https://github.com/dcramer/mock-django/blob/master/mock_django/query.py I think you can then just patch you models objects function to return one of these QuerySetMock objects that you have set up to return something expected!
For this I use Django’s .none() function.
For example:
class Location(models.Model):
name = models.CharField(max_length=100)
mock_locations = Location.objects.none()
This is the method used frequently in Django’s own internal test cases. Based on comments in the code
Calling none() will create a queryset that never returns any objects and no
+query will be executed when accessing the results. A qs.none() queryset
+is an instance of ``EmptyQuerySet``.
For an empty Queryset, I’d go simply for using none as keithhackbarth has already stated.
However, to mock a Queryset that will return a list of values, I prefer to use a Mock with a spec of the Model’s manager. As an example (Python 2.7 style – I’ve used the external Mock library), here’s a simple test where the Queryset is filtered and then counted:
from django.test import TestCase
from mock import Mock
from .models import Example
def queryset_func(queryset, filter_value):
"""
An example function to be tested
"""
return queryset.filter(stuff=filter_value).count()
class TestQuerysetFunc(TestCase):
def test_happy(self):
"""
`queryset_func` filters provided queryset and counts result
"""
m_queryset = Mock(spec=Example.objects)
m_queryset.filter.return_value = m_queryset
m_queryset.count.return_value = 97
result = func_to_test(m_queryset, '__TEST_VALUE__')
self.assertEqual(result, 97)
m_queryset.filter.assert_called_once_with(stuff='__TEST_VALUE__')
m_queryset.count.assert_called_once_with()
However, to fulfil the question, instead of setting a return_value for count, this could easily be adjusted to be a list of model instances returned from all.
Note that chaining is handled by setting the filter to return the mocked queryset:
m_queryset.filter.return_value = m_queryset
This would need to be applied for any queryset methods used in the function under test, e.g. exclude, etc.
Try out the django_mock_queries library that lets you mock out the database access, and still use some of the Django query set features like filtering.
Full disclosure: I contributed some features to the project.
You can mock like this:
@patch('django.db.models.query.QuerySet')
def test_returning_distinct_records_for_city(self, mock_qs):
self.assertTrue(mock_qs.called)
Have you looked into FactoryBoy? https://factoryboy.readthedocs.io/en/latest/orms.html
It’s a fixtures replacement tool with support for the django orm – factories basically generate orm-like objects (either in memory or in a test database).
Here’s a great article for getting started: https://www.caktusgroup.com/blog/2013/07/17/factory-boy-alternative-django-testing-fixtures/
I have a utility function in my Django project, it takes a queryset, gets some data from it and returns a result. I’d like to write some tests for this function. Is there anyway to ‘mock’ a QuerySet? I’d like to create an object that doesn’t touch the database, and i can provide it with a list of values to use (i.e. some fake rows) and then it’ll act just like a queryset, and will allow someone to do field lookups on it/filter/get/all etc.
Does anything like this exist already?
Not that I know of, but why not use an actual queryset? The test framework is all set up to allow you to create sample data within your test, and the database is re-created on every test, so there doesn’t seem to be any reason not to use the real thing.
Of course you can mock a QuerySet, you can mock anything.
You can create an object yourself, and give it the interface you need, and have it return any data you like. At heart, mocking is nothing more than providing a “test double” that acts enough like the real thing for your tests’ purposes.
The low-tech way to get started is to define an object:
class MockQuerySet(object):
pass
then create one of these, and hand it to your test. The test will fail, likely on an AttributeError. That will tell you what you need to implement on your MockQuerySet. Repeat until your object is rich enough for your tests.
One first advice would be to split the function in two parts, one that creates the queryset
and one that manipulates the output of it. In this way testing the second part is straightforward.
For the database problem, I investigated if django uses sqlite-in-memory and I found out that
recent version of django uses the sqlite -in-memory database, from The django unittest page:
When using the SQLite database engine the tests will by default use an
in-memory database (i.e., the database will be created in memory,
bypassing the filesystem entirely!).
Mocking the QuerySet object will not make you exercise its full logic.
I am having the same issue, and it looks like some nice person has written a library for mocking QuerySets, it is called mock-django and the specific code you will need is here https://github.com/dcramer/mock-django/blob/master/mock_django/query.py I think you can then just patch you models objects function to return one of these QuerySetMock objects that you have set up to return something expected!
For this I use Django’s .none() function.
For example:
class Location(models.Model):
name = models.CharField(max_length=100)
mock_locations = Location.objects.none()
This is the method used frequently in Django’s own internal test cases. Based on comments in the code
Calling none() will create a queryset that never returns any objects and no
+query will be executed when accessing the results. A qs.none() queryset
+is an instance of ``EmptyQuerySet``.
For an empty Queryset, I’d go simply for using none as keithhackbarth has already stated.
However, to mock a Queryset that will return a list of values, I prefer to use a Mock with a spec of the Model’s manager. As an example (Python 2.7 style – I’ve used the external Mock library), here’s a simple test where the Queryset is filtered and then counted:
from django.test import TestCase
from mock import Mock
from .models import Example
def queryset_func(queryset, filter_value):
"""
An example function to be tested
"""
return queryset.filter(stuff=filter_value).count()
class TestQuerysetFunc(TestCase):
def test_happy(self):
"""
`queryset_func` filters provided queryset and counts result
"""
m_queryset = Mock(spec=Example.objects)
m_queryset.filter.return_value = m_queryset
m_queryset.count.return_value = 97
result = func_to_test(m_queryset, '__TEST_VALUE__')
self.assertEqual(result, 97)
m_queryset.filter.assert_called_once_with(stuff='__TEST_VALUE__')
m_queryset.count.assert_called_once_with()
However, to fulfil the question, instead of setting a return_value for count, this could easily be adjusted to be a list of model instances returned from all.
Note that chaining is handled by setting the filter to return the mocked queryset:
m_queryset.filter.return_value = m_queryset
This would need to be applied for any queryset methods used in the function under test, e.g. exclude, etc.
Try out the django_mock_queries library that lets you mock out the database access, and still use some of the Django query set features like filtering.
Full disclosure: I contributed some features to the project.
You can mock like this:
@patch('django.db.models.query.QuerySet')
def test_returning_distinct_records_for_city(self, mock_qs):
self.assertTrue(mock_qs.called)
Have you looked into FactoryBoy? https://factoryboy.readthedocs.io/en/latest/orms.html
It’s a fixtures replacement tool with support for the django orm – factories basically generate orm-like objects (either in memory or in a test database).
Here’s a great article for getting started: https://www.caktusgroup.com/blog/2013/07/17/factory-boy-alternative-django-testing-fixtures/