Solved: {“non_field_errors”: [“Expected a list of items but got type “dict”.”]}

Developers working with Django REST Framework (DRF) often encounter serialization issues when handling nested objects or ManyToMany relationships in APIs. One of the common errors is: “{“non_field_errors”: [“Expected a list of items but got type “dict”.”]}”. This error typically arises when DRF expects a list of items but instead receives a dictionary. Understanding why this happens and how to fix it ensures smoother API development and prevents data serialization issues.

Understanding the Error: Why Does Django REST Framework Expect a List?

🔹 Django REST Framework serializers are designed to handle different types of data structures, including single objects, lists of objects, and nested relationships.
🔹 When using a ManyToManyField or ForeignKey with a nested serializer, DRF expects a list of objects for ManyToMany fields but a dictionary for ForeignKey fields.
🔹 If the API receives a dictionary instead of a list where a ManyToMany field is expected, it triggers the error: “Expected a list of items but got type “dict”.”
🔹 This issue commonly occurs when working with nested serializers, bulk data submissions, or incorrect request payload formatting.

Identifying whether the issue is caused by incorrect serializer definitions or request payload structure is the first step toward solving it.

How to Fix the “Expected a List of Items but Got Type Dict” Error?

Consider a ManyToMany relationship where an Author can have multiple Books, as defined in models.py:

from django.db import models

class Book(models.Model):
    title = models.CharField(max_length=255)

class Author(models.Model):
    name = models.CharField(max_length=255)
    books = models.ManyToManyField(Book)

In the corresponding serializers.py, using a nested serializer without setting many=True will cause the error:

from rest_framework import serializers
from .models import Book, Author

class BookSerializer(serializers.ModelSerializer):
    class Meta:
        model = Book
        fields = '__all__'

class AuthorSerializer(serializers.ModelSerializer):
    books = BookSerializer()

    class Meta:
        model = Author
        fields = '__all__'

If an API request is made with the following payload:

{
    "name": "John Doe",
    "books": {"title": "Django for Beginners"}
}

DRF will return the error “Expected a list of items but got type “dict”.” because books is a ManyToManyField and expects a list.

class AuthorSerializer(serializers.ModelSerializer):
    books = BookSerializer(many=True)

    class Meta:
        model = Author
        fields = '__all__'

{
    "name": "John Doe",
    "books": [
        {"title": "Django for Beginners"},
        {"title": "Advanced Django"}
    ]
}

class AuthorSerializer(serializers.ModelSerializer):
    books = serializers.PrimaryKeyRelatedField(queryset=Book.objects.all(), many=True)

    class Meta:
        model = Author
        fields = '__all__'

{
    "name": "John Doe",
    "books": [1, 2]
}

class AuthorSerializer(serializers.ModelSerializer):
    books = BookSerializer(many=True)

    class Meta:
        model = Author
        fields = '__all__'

    def create(self, validated_data):
        books_data = validated_data.pop('books')
        author = Author.objects.create(**validated_data)
        for book_data in books_data:
            book, created = Book.objects.get_or_create(**book_data)
            author.books.add(book)
        return author

class ChapterSerializer(serializers.ModelSerializer):
    book = serializers.PrimaryKeyRelatedField(queryset=Book.objects.all())

    class Meta:
        model = Chapter
        fields = '__all__'

{
    "title": "Chapter One",
    "content": "Introduction to Django",
    "book": 1
}