Saptak's Blog Posts

Handling nested serializer validations in Django Rest Framework

Posted: 2020-05-03T01:22:50+05:30

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": "",
  "source_language": {
    "code": "en",
    "name": "English",
    "direction": "ltr",
    "web_url": "http:/",
    "url": ""
  "web_url": "",
  "url": "",
  "components_list_url": "",
  "repository_url": "",
  "statistics_url": "",
  "changes_list_url": "",
  "languages_url": ""

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.


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": "",
  "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.


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(
    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.

Creating Custom Whoosh Plugin

Posted: 2020-04-19T13:16:52+05:30

Recently, while trying to work on a query parser feature in Weblate, I came across this search engine library called Whoosh. It provides certain nice features like indexing of text, parsing of search queries, scoring algorithms, etc. One good thing about this library is most of these features are customizable and extensible.

Now, the feature I was trying to implement is an exact search query. An exact search query would behave in a way such that the backend would search for an exact match of any query text provided to it instead of the normal substring search. Whoosh provides a plugin for regex, which can be accessed via whoosh.qparser.RegexPlugin(). So we can technically go about writing a regex to do the exact match. But a regex search will have worse performance than a simple string comparison.

So, one of the ways of doing a new kind of query parsing is creating a custom whoosh plugin. And that's what this blog is going to be about.

Simple Whoosh Plugin

In some cases, you will probably not need a complicated plugin, but just want to extend the feature of an existing plugin to match a different kind of query. For example, let's say you want to extend the ability of SingleQuotePlugin to parse queries wrapped in either single-quotes or double-quotes.

class QuotePlugin(whoosh.qparser.SingleQuotePlugin):
    """Single and double quotes to specify a term."""
    expr = r"(^|(?<=\W))['\"](?P<text>.*?)['\"](?=\s|\]|[)}]|$)"

In the above example, QuotePlugin extends the already existing SingleQuotePlugin class. It just overrides the expression to parse the query. The expression, mentioned in the variable expr is usually a regex expression with ?P<text> part denoting the TermQuery. A TermQuery is the final term/terms searched for in the database. So in the above regex, we say to parse any query such that the TermQuery is wrapped in between single-quotes or double-quotes.

Query Class

A query class is the class, whose instance the final parsed term will be. Unless otherwise mentioned, it's usually <Term>. So if we want our plugin to parse the query and show it as an instance of a custom class, we need to define a custom query class.

class Exact(whoosh.query.Term):
    """Class for queries with exact operator."""


So, as you can say, we can just have a simple class just extending whoosh.query.Term so that while checking the parsed terms, we can get is as an instance of Exact. That will help us differentiate the query from a normal Term instance.

Custom Whoosh Plugin

After writing the query class, we will need to write the custom plugin class.

class ExactPlugin(whoosh.qparser.TaggingPlugin):
    """Exact match plugin with quotes to specify an exact term."""

    class ExactNode(whoosh.qparser.syntax.TextNode):
        qclass = Exact

        def r(self):
            return "Exact %r" % self.text

    expr = r"\=(^|(?<=\W))(['\"]?)(?P<text>.*?)\2(?=\s|\]|[)}]|$)"
    nodetype = ExactNode

In the above example, unlike the simple case, we extend TaggingPlugin instead of any other pre-defined plugin. Most of the pre-defined plugins in whoosh also extend TaggingPlugin. So it is a good fit as a parent class.

Then, we create a ExactNode class. This we will assign to the node type for the custom plugin. A node type class basically defines the query class to be used in this custom plugin, along with various representations and properties of the parsed node. qclass will have the query class created before to denote the Exact instance to the final parsed term.

Apart from that, we have the expr which contains the regex just like in the simple example to parse the query term.


After creating the custom plugin, you can:

  • add this plugin to the list of plugins defined in the whoosh query parser class
  • use the query class to make an isinstance() check when making database queries
  • check for the node type in the different nodes used by the parser