python - Order of Serializer Validation in Django REST Framework

Question

Situation

While working with validation in the Django REST Framework's ModelSerializer, I have noticed that the Meta.model fields are always validated, even when it does not necessarily make sense to do so. Take the following example for a User model's serialization:

1. I have an endpoint that creates a user. As such, there is a password field and a confirm_password field. If the two fields do not match, the user cannot be created. Likewise, if the requested username already exists, the user cannot be created.
2. The user POSTs improper values for each of the fields mentioned above
3. An implementation of validate has been made in the serializer (see below), catching the non-matching password and confirm_password fields

Implementation of validate:

def validate(self, data):
    if data['password'] != data.pop('confirm_password'):
        raise serializers.ValidationError("Passwords do not match")
    return data

Problem

Even when the ValidationError is raised by validate, the ModelSerializer still queries the database to check to see if the username is already in use. This is evident in the error-list that gets returned from the endpoint; both the model and non-field errors are present.

Consequently, I would like to know how to prevent model validation until after non-field validation has finished, saving me a call to my database.

Attempt at solution

I have been trying to go through the DRF's source to figure out where this is happening, but I have been unsuccessful in locating what I need to override in order to get this to work.

Answer 1

Since most likely your username field has unique=True set, Django REST Framework automatically adds a validator that checks to make sure the new username is unique. You can actually confirm this by doing repr(serializer()), which will show you all of the automatically generated fields, which includes the validators.

Validation is run in a specific, undocumented order

1. Field deserialization called (serializer.to_internal_value and field.run_validators)
2. serializer.validate_[field] is called for each field
3. Serializer-level validators are called (serializer.run_validation followed by serializer.run_validators)
4. serializer.validate is called

So the problem that you are seeing is that the field-level validation is called before your serializer-level validation. While I wouldn't recommend it, you can remove the field-level validator by setting extra_kwargs in your serilalizer's meta.

class Meta:
    extra_kwargs = {
        "username": {
            "validators": [],
        },
    }

You will need to re-implement the unique check in your own validation though, along with any additional validators that have been automatically generated.