Django Unit Test - using factory_boy build() on a Model with Many-To-Many relationship

Question

I’m working on writing unit tests for a DRF project using pytest and factory_boy. I’m running into issues with many-to-many relationships. Specifically, when I try to use .build() in my unit tests, DRF attempts to access the M2M field which requires a saved object, leading to errors.

tests_serializers.py

def test_serialize_quality_valid_data(self): 
 user = UserFactory.build() 
 quality = QualityFactory.build(created_by=user) 
 serializer = QualitySerializer(quality) 
 data = serializer.data 
 assert data["num"] == quality.num 

error:

FAILED quality/tests/tests_serializers.py::TestQualitySerializer::test_serialize_quality_valid_data - ValueError: "<Quality: Quality object (None)>" needs to have a value for field "id" before this many-to-many relationship can be used.

model.py

class QualityTag(ExportModelOperationsMixin("quality_tag"), models.Model): 
 name = models.CharField(max_length=64, unique=True) 
 description = models.TextField() 
class Quality(ExportModelOperationsMixin("quality"), models.Model): 
 num = models.IntegerField() 
 title = models.CharField(max_length=64) 
 ... 
 tags = models.ManyToManyField(QualityTag, related_name="qualities", blank=True) 

factories.py

class QualityTagFactory(DjangoModelFactory): 
 class Meta: 
 model = QualityTag 
 name = factory.Sequence(lambda n: f"Quality Tag {n}") 
class QualityFactory(factory.django.DjangoModelFactory): 
 class Meta: 
 model = Quality 
 num = factory.Faker("random_int", min=1, max=999) 
 @factory.post_generation 
 def tags(self, create, extracted, **kwargs): 
 if not create: 
 return 
 if extracted: 
 for tag in extracted: 
 self.tags.add(tag) 

serializers.py

class QualitySerializer(serializers.ModelSerializer): 
tags = QualityTagDetailSerializer(many=True) 
created_by = UserProfileSerializer() 
 updated_by = UserProfileSerializer() 
class Meta: model = Quality 
 fields = "__all__" 
read_only_fields = ["quality_num", "tags", "created_by", "updated_by"] 

I’ve been advised to switch to .create() instead of .build(), but I’d prefer to keep this as a pure unit test if possible (I believe it may become an integration test if I use build()?) I have tried converting it to using factory_boy .create(), however, I want to keep it as a simple unit test.

I have tried converting it to using factory_boy .create(), however, I want to keep it as a simple unit test.

Answer 1

As I know when you use factory_boy.build(), the object exists only in memory without a database ID. Django's M2M fields require a saved object (with an ID) to function properly. So, you can test your serializer logic without hitting the database by mocking the M2M field, something like this:

def test_serialize_quality_with_tags(self):
 user = UserFactory.build()
 quality = QualityFactory.build(created_by=user)
 
 # Create tag objects in memory
 tag1 = QualityTagFactory.build(name="Tag 1")
 tag2 = QualityTagFactory.build(name="Tag 2")
 mock_tags_data = [tag1, tag2]
 
 with patch.object(quality, 'tags') as mock_tags:
 # Mock the M2M field to return our test data
 mock_tags.all.return_value = mock_tags_data
 mock_tags.__iter__ = lambda x: iter(mock_tags_data)
 
 serializer = QualitySerializer(quality)
 data = serializer.data
 
 assert data["num"] == quality.num
 assert len(data["tags"]) == 2
 assert data["tags"][0]["name"] == "Tag 1"

check this for more details: https://factoryboy.readthedocs.io/en/stable/reference.html#id9