Django REST Framework makes it easy to quickly build REST APIs for Django models. The ModelSerializer class provides a convenient way to serialize Django models to and from JSON. Often, you may need to add extra fields to the serialized representation that are not part of the model itself. In this post, we’ll explore a few different approaches to accomplishing this.
The Need for Extra Fields
There are many reasons why you may want to include additional computed fields in a ModelSerializer beyond just the database model fields. Some examples include:
- Adding resource URLs
- Incorporating extra calculated data
- Integrating metadata
- Adding nested relationships
Having this flexibility helps produce rich, descriptive JSON responses without cluttering up the actual database models.
Computed Properties
One straightforward way to add extra fields is by utilizing the existing .field options on the ModelSerializer class. For example:
class ArticleSerializer(serializers.ModelSerializer):
url = serializers.CharField(source='get_absolute_url', read_only=True)
class Meta:
model = Article
fields = ['id', 'title', 'content', 'url']Here, we have added a url field that calls the get_absolute_url() method on each Article instance to return the URL. This keeps related logic in the model class while exposing the field through the API.
This approach works well for simple computed properties, but it can get messy if more complex field computation is needed.
Custom Fields
For more customization, you can declare explicit serializerFields tied to model attributes:
class ArticleSerializer(serializers.ModelSerializer):
url = serializers.SerializerMethodField('get_url')
def get_url(self, article):
return article.get_absolute_url()
class Meta:
model = Article
fields = ['id', 'title', 'content', 'url']Now the URL generation is encapsulated in a reusable method. The `SerializerMethodField` handles calling that method automatically during serialization.
This technique is useful for reformatting data or accessing related model fields. However, it requires declaring each custom field individually.
Extra Field Arguments
As another option, the `.extra_kwargs` attribute allows passing additional kwargs to the default model fields.
For example:
class ArticleSerializer(serializers.ModelSerializer):
class Meta:
model = Article
fields = ['id', 'title', 'content']
extra_kwargs = {
'url': {'source': 'get_absolute_url'}
}This approach essentially merges custom field attributes with the default model ones. It condenses the field configuration while minimizing boilerplate code.
Conclusion
Django REST Framework offers developers several different techniques for adding extra fields beyond the base model ones to ModelSerializer classes. Typical use cases include adding URLs, computed data, metadata or related fields.
By leveraging serialized field options, custom methods and extra keyword arguments, you can augment your API representations while keeping a clean separation of concerns between database models and API outputs. Experiment to discover which approach best suits your REST API design and maintenance needs.
