Order of Serializer Validation in Django REST Framework

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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.

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