How To Filter A Nested Serializer In Django Rest Framework?

Last Updated : 04 Oct, 2024

When building APIs with Django Rest Framework (DRF), nested serializers are commonly used to represent relationships between models. A nested serializer allows us to include data from related models within a serializer. However, there are instances where we might want to filter the data returned by the nested serializer based on certain conditions. In this article, we'll explore what nested serializers are and how to apply filtering techniques to them in Django Rest Framework.

Table of Content

What is a Nested Serializer?
Filter a Nested Serializer in Django Rest Framework
Apply Filtering to the Nested Serializer

Custom Method on the Serializer
Pass Filtered QuerySet in the View
Using DRF Filters for Nested Data:

What is a Nested Serializer?

A nested serializer allows us to represent complex relationships between models by embedding one serializer within another. For instance, if we have a Post model related to a Comment model, a nested serializer can include the Comment data within the Post serializer.

Example:

Python

from django.db import models

class Post(models.Model):
   	title = models.CharField(max_length=100)
	content = models.TextField()
    
class Comment(models.Model):
    post = models.ForeignKey(Post, related_name='comments', on_delete=models.CASCADE)
	text = models.TextField()
    created_at = models.DateTimeField(auto_now_add=True)

Here, the Comment model has a foreign key to the Post model, indicating a one-to-many relationship. Using a nested serializer, we can display comments alongside posts

Filter a Nested Serializer in Django Rest Framework

Step 1: Create a New Django Project

First, create a new Django project and app:

django-admin startproject blog
cd blog
python manage.py startapp posts

Add 'posts' and 'rest_framework' to the INSTALLED_APPS in settings.py.

Python

# ...

INSTALLED_APPS = [
  	# ...
  	'posts',
  	'rest_framework'
]

Step 2: Define Models

We will use the Post and Comment models from above. Add them to posts/models.py:

Python

from django.db import models


class Post(models.Model):
    title = models.CharField(max_length=100)
    content = models.TextField()


class Comment(models.Model):
    post = models.ForeignKey(
        Post, related_name='comments', on_delete=models.CASCADE)
    text = models.TextField()
    is_approved = models.BooleanField(default=False)

Run Migrations

python manage.py makemigrations
python manage.py migrate

Step 3: Create Serializers

We’ll create serializers to handle Post and Comment models. The PostSerializer will nest the CommentSerializer.

posts/serializers.py

Python

from rest_framework import serializers
from .models import Post, Comment


class CommentSerializer(serializers.ModelSerializer):
    class Meta:
        model = Comment
        fields = ['text', 'is_approved']


class PostSerializer(serializers.ModelSerializer):
    comments = CommentSerializer(many=True)

    class Meta:
        model = Post
        fields = ['title', 'content', 'comments']

Step 4: Adding Data to the Database

To populate the database, use the following steps to add some Post and Comment data.

Open the Django shell:

python manage.py shell

Create some Post instances and Add Comment instances related to the Post instances.

Python

from posts.models import Post, Comment

post1 = Post.objects.create(title="Post 1", content="Content for Post 1")
post2 = Post.objects.create(title="Post 2", content="Content for Post 2")
post3 = Post.objects.create(title="Post 3", content="Content for Post 3")

# Comments for Post 1
Comment.objects.create(post=post1, text="Comment 1 on Post 1", is_approved=True)
Comment.objects.create(post=post1, text="Comment 2 on Post 1", is_approved=False) 

# Comments for Post 2
Comment.objects.create(post=post2, text="Comment 1 on Post 2", is_approved=True)
Comment.objects.create(post=post2, text="Comment 2 on Post 2", is_approved=True) 

# Comments for Post 3
Comment.objects.create(post=post3, text="Comment 1 on Post 3", is_approved=False)
Comment.objects.create(post=post3, text="Comment 2 on Post 3", is_approved=True)

Now, we have three Post instances, each with two Comment instances (some approved and some not approved).

Apply Filtering to the Nested Serializer

Now, let’s apply filtering to the comments field inside the PostSerializer. Instead of filtering by date, we will filter comments based on whether they are approved (is_approved=True).

1. Custom Method on the Serializer

One way to filter nested data is by overriding the to_representation method of the parent serializer and applying the filter logic within that method.

posts/serializers.py

Python

class PostSerializer(serializers.ModelSerializer):
    comments = serializers.SerializerMethodField()

    class Meta:
        model = Post
        fields = ['title', 'content', 'comments']

    def get_comments(self, obj):
        # Filter only approved comments
        approved_comments = obj.comments.filter(is_approved=True)
        return CommentSerializer(approved_comments, many=True).data

In this method, we use a custom method to filter the nested serializer: The PostSerializer has the filtering logic in the get_comments method, which filters approved comments only. No additional logic is needed in the view for this method.

posts/views.py

Python

from rest_framework import generics
from .models import Post
from .serializers import PostSerializer

class PostListView(generics.ListAPIView):
    queryset = Post.objects.all()
    serializer_class = PostSerializer

blog/urls.py

Python

from django.contrib import admin
from django.urls import path
from posts.views import PostListView

urlpatterns = [
    path('admin/', admin.site.urls),
    path('posts/', PostListView.as_view(), name='post-list'),
]

Run the Server

python manage.py runserver

URL: http://127.0.0.1:8000/posts/
This will return only approved comments for each post.

Filter nested data in Serializer in Django Rest Framework

2. Pass Filtered QuerySet in the View

Another approach is to filter the comments in the view and pass the filtered queryset to the serializer. Instead of trying to assign a filtered queryset to post.comments, we can handle the filtering during serialization and use prefetch_related() to optimize the query. Here, we filter the nested serializer directly in the view before passing it to the serializer:

Python

# posts/views.py
from rest_framework import generics
from .models import Post
from .serializers import PostSerializer
from django.db.models import Prefetch
from .models import Comment

class FilteredPostListView(generics.ListAPIView):
    serializer_class = PostSerializer

    def get_queryset(self):
        # Use prefetch_related to fetch comments with the filter applied
        return Post.objects.prefetch_related(
            Prefetch('comments', queryset=Comment.objects.filter(is_approved=True))
        )

urls.py

Python

from django.contrib import admin
from django.urls import path
from posts.views import FilteredPostListView


urlpatterns = [
    path('admin/', admin.site.urls),
    path('posts-filtered/', FilteredPostListView.as_view(), name='filtered-post-list'),
]

Run the Server

python manage.py runserver

URL: http://127.0.0.1:8000/posts-filtered/
This will also return only approved comments for each post, with filtering happening in the view.

Filtering Nested Serializer in Django Rest Framework

3. Using DRF Filters for Nested Data:

If we want to add more flexibility to our filtering, we can use query parameters along with Django’s filtering capabilities. First, install the django-filter package:

pip install django-filter

Update the settings to enable filtering:

settings.py

Python

REST_FRAMEWORK = {
    'DEFAULT_FILTER_BACKENDS': ['django_filters.rest_framework.DjangoFilterBackend']
}

In views.py, we can now use filter backends to dynamically filter data based on query parameters. We'll enable dynamic filtering using query parameters for flexibility:

posts/views.py

Python

from django_filters.rest_framework import DjangoFilterBackend
from rest_framework import generics
from .models import Post
from .serializers import PostSerializer

class PostListFilterView(generics.ListAPIView):
    queryset = Post.objects.all()
    serializer_class = PostSerializer
    filter_backends = [DjangoFilterBackend]
    filterset_fields = {
        'comments__is_approved': ['exact']
    }

blog/urls.py

Python

from django.contrib import admin
from django.urls import path
from posts.views import PostListFilterView


urlpatterns = [
    path('admin/', admin.site.urls),
    path('posts-dynamic-filter/', PostListFilterView.as_view(), name='post-list-filter'),
]

Run the Server

python manage.py runserver

URL: http://127.0.0.1:8000/posts-dynamic-filter/?comments__is_approved=true
This will dynamically filter posts based on the approved status of comments.

Conclusion

Each method provides a different way to filter nested serializers in Django Rest Framework. Depending on our needs, we can choose to filter directly in the serializer, within the view, or using query parameters via Django's filtering system.