Django Ninja Schemas and Many To Many

On a recent project, I am using Django Ninja instead of Django Rest Framework mainly because I wanted to have better control over typing and also because it’s slightly easier to implement.

When writing the tests and the API for the project, it was clear that the ManyToMany relationship between the two models failed the validation.

The Code

These are the models. As you can see, we have a HabitCategory and a Habit. Each Habit can have a Category.

class HabitCategory(models.Model):
    title = models.CharField(max_length=255)
    icon = models.ImageField(null=True, blank=True)


class Habit(models.Model):
    created_at = models.DateTimeField(blank=False, auto_now_add=True)
    user = models.ForeignKey(User, on_delete=models.CASCADE)
    title = models.CharField(max_length=1000)
    post = models.TextField(blank=True)
    category = models.ManyToManyField(HabitCategory, blank=True)
    visibility = models.CharField(
        max_length=10,
        choices=HabitVisibilityChoices.choices,
        default=HabitVisibilityChoices.PUBLIC,
    )

The test itself is pretty basic. It makes a GET request and tries to assert that the representation of the object is correct. It might be worth mentioning that I have created fixtures to create a test object, user and Django client.

def test_get_habit(habit_with_updates_and_interactions, django_client):
    habit_id = habit_with_updates_and_interactions.id
    response = django_client.get(reverse("api-v1:habit", kwargs={"pk": habit_id}))

    assert response.status_code == 200
    habit = response.json()
    assert habit.title == "Think about my parents"

Finally, let’s have a look at the view and the Schema:

class HabitSchema(ModelSchema):
    user: UserSchema
    category: List[str]
    total_updates: int
    total_thumbs_up: int
    total_comments: int

    class Config:
        model = Habit
        model_fields = [
            "id",
            "created_at",
            "user",
            "title",
            "post",
            "visibility",
            "category",
        ]

@router.get(url_name="habit", path="/habit/{pk}", response=HabitSchema)
def get_habit(request: HttpRequest, pk: int):
    return get_object_or_404(Habit, pk=pk, user=request.user)

I am using a UserSchema and a HabitCategorySchema, which should give me just the user.username and the habit_category.title. But when the tests run, the following validation error pops up:

pydantic.error_wrappers.ValidationError: 1 validation error for NinjaResponseSchema
response -> category -> 0
  field required (type=value_error.missing)

Okay, so it seems that the category doesn’t have a title. This isn’t surprising since the category has a ManyToMany relationship.

Addressing the problem

We can remove the category: HabitCategorySchema mention in the Schema, and the tests will pass, but the category isn’t what we expect.

# Updated Schema without category
class HabitSchema(ModelSchema):
	user: UserSchema
    total_updates: int
    total_thumbs_up: int
    total_comments: int

    class Config:
        model = Habit
        model_fields = [
            "id",
            "created_at",
            "user",
            "title",
            "post",
            "visibility",
            "category",
        ]

# Response from the API
{
    'id': 5,
    'created_at': '2022-08-16T09:11:48.041358+00:00',
    'user': {
        'username': 'batman'
    },
    'title': 'Think about my parents',
    'post': 'They were awesome',
    'visibility': 'private',
    'category': [14],
    'total_updates': 2,
    'total_thumbs_up': 3,
    'total_comments': 2
}

As you can see, category is now available, but it’s returning a list and the HabitCategory id. But what we want is the title from this specific category.

The trick is to write a static method to fetch the category title.

class HabitSchema(ModelSchema):
	user: UserSchema
	category: List[str]
    total_updates: int
    total_thumbs_up: int
    total_comments: int

    class Config:
        model = Habit
        model_fields = [
            "id",
            "created_at",
            "user",
            "title",
            "post",
            "visibility",
            "category",
        ]

	@staticmethod
	def resolve_category(obj):
		return [category.title for category in obj.category.all()]

This will iterate over all the object categories and return the title for us. Note that we also need to add category: List[str] to the Schema. When we run the tests, the API will now return:

{
    'id': 5,
    'created_at': '2022-08-16T09:23:19.361886+00:00',
    'user': {
        'username': 'batman'
    },
    'title': 'Think about my parents',
    'post': 'They were awesome',
    'visibility': 'private',
    'category': [
        'Family'
    ],
    'total_updates': 2,
    'total_thumbs_up': 3,
    'total_comments': 2
}

Other alternatives

This article wouldn’t be completed if I didn’t show other ways to achieve a similar result. But you could use either of these alternatives and get good results. In my case, I wanted the category to be a list of strings.

Using the HabitCategorySchema

We could pass the HabitCategorySchema to the Habit Schema, and it would work. For example:

class HabitSchema(ModelSchema):
    created_at: datetime.datetime
    user: UserSchema
    title: str
    post: str
    category: List[HabitCategorySchema]
    visibility: str
    total_updates: int
    total_thumbs_up: int
    total_comments: int

    class Config:
        model = Habit
        model_fields = [
            "id",
            "created_at",
            "user",
            "title",
            "post",
            "visibility",
            "category",
        ]

# The API returns:
{
    'id': 5,
    'created_at': '2022-08-16T09:28:25.872980+00:00',
    'user': {
        'username': 'batman'
    },
    'title': 'Think about my parents',
    'post': 'They were awesome',
    'visibility': 'private',
    'category': [
        {'title': 'Family'}
    ],
    'total_updates': 2,
    'total_thumbs_up': 3,
    'total_comments': 2
}

The response is a bit more complex, but maybe that’s okay for your use case.

Using a one-liner(ish)

Depending on your model, you may be able to use the create_schema function to make Django Ninja create the Schema for us. Under the hood, the ModelSchema class uses this function to generate the Schema, and we can also remove a lot of our code and use the

HabitSchema = create_schema(Habit, depth=1)

I couldn’t make create_schema work for this particular case and decided not to spend more time on this approach.

I hope this helps!

References:

• Django Ninja
• Docs: using create_schema
• Issue: Flatten Many to Many