Handling nested serializer validations in Django Rest Framework

I understand that the title of this post is a little confusing. Recently, while working on the Projects API in Weblate, I came across an interesting issue. The Projects API in Weblate allowed you to get an attribute called source_language. Every project has only one source_language and in the API, it was a read-only property.

{
  "name": "master_locales",
  "slug": "master_locales",
  "web": "https://example.site",
  "source_language": {
    "code": "en",
    "name": "English",
    "direction": "ltr",
    "web_url": "http:/example.site/languages/en/",
    "url": "http://example.site/api/languages/en/"
  },
  "web_url": "http://example.site/projects/master_locales/",
  "url": "http://example.site/api/projects/master_locales/",
  "components_list_url": "http://example.site/api/projects/master_locales/components/",
  "repository_url": "http://example.site/api/projects/master_locales/repository/",
  "statistics_url": "http://example.site/api/projects/master_locales/statistics/",
  "changes_list_url": "http://example.site/api/projects/master_locales/changes/",
  "languages_url": "http://example.site/api/projects/master_locales/languages/"
}

As you can see, unlike the other relational fields, it's not a HyperLinkedIdentityField. It uses the nested language serializer to show all the attributes of the source_language.

Now, previously, when a project was created via API, a default language was always assigned to the project and there was no way to define the source_language while creating the project via API.

Problem?

Doing GET on Language Serializer when sending POST on Project Serializer

So we needed to add the feature to define the source_language of the project when we send a POST request to the Project API. And also edit the project via API to update the source_language. So, to use the same serializer, the request body for the POST request would look something like this:

{
  "name": "master_locales",
  "slug": "master_locales",
  "web": "https://example.site",
  "source_language": {
    "code": "ru",
    "name": "Russian",
    "direction": "ltr",
  }
}

Now, in general, we would have a python serializer like this:

class LanguageSerializer(serializers.ModelSerializer):
    web_url = AbsoluteURLField(source="get_absolute_url", read_only=True)
    class Meta:
        model = Language
        fields = ("code", "name", "direction", "web_url", "url")
        extra_kwargs = {
            "url": {"view_name": "api:language-detail", "lookup_field": "code"}
        }


class ProjectSerializer(serializers.ModelSerializer):
    source_language = LanguageSerializer(required=False)
    # ...
    # Other parts of the serializer

The problem with having code like this is, when the ProjectSerializer gets a request like shown above and tries to validate the data in the request, it also validates the LanguageSerializer part. The LanguageSerializer part whenever it gets data, it will automatically try to validate the data. The code property of Language model has a unique constraint. So, when LanguageSerializer tries to validate

{
    "code": "ru",
    "name": "Russian",
    "direction": "ltr",
}

it will throw an error "This field must be unique" for code property in case a language with codename ru already exists in the database.

Solution

So there are few steps to get this done.

Remove validators from code field

extra_kwargs = {
    "url": {"view_name": "api:language-detail", "lookup_field": "code"},
    "code": {"validators": []},
}

Add "code": {"validators": []} to the extra_kwargs to remove the validator from the LanguageSerializer on every data request it receives.

Add manual validation for code field

Removing validator will also remove the validation while doing POST request. Now, the LanguageSerializer in Weblate specifically doesn't support POST, but in any case, you would manually need to add a validation function to the LanguageSerializer so if someone checks for validity before adding language, it throws an error. To do that, add a function validate_code like this:

def validate_code(self, value):
    check_query = Language.objects.filter(code=value)
    if check_query.exists() and not (
        isinstance(self.parent, ProjectSerializer)
        and self.field_name == "source_language"
    ):
        raise serializers.ValidationError(
            "Language with this Language code already exists."
        )
    if not check_query.exists():
        raise serializers.ValidationError(
            "Language with this language code was not found."
        )
    return value

Note: The name of the function must be validate_{field_name} when you are trying to validate a field based on how DRF handles validation.

Overwrite create() in ProjectSerializer

Finally, we would want to overwrite the create() function of ProjectSerializer to:

• Validate source_language data using the above validation to check if the language with that code exists
• Modify source_language key of the validated_data to have the Language model object rather than the dictionary, so it can be used to create a project with the foreign key.
• Lastly, create a project with the new validata_data

The code would look something like this:

def create(self, validated_data):
    source_language_validated = validated_data.get("source_language")
    if source_language_validated:
        validated_data["source_language"] = Language.objects.get(
            code=source_language_validated.get("code")
        )
    project = Project.objects.create(**validated_data)
    return project

And now, if you create a project, using the source_language key, you can define the source language for the project while using the Project API. There might be several other ways to go about it. But this is one of the ways I found works.

Also, this feature is now live in Weblate 4.* versions which allows you to define the source_language via the API.