ModelSerializer for model-backed APIs in Django - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this ModelSerializer validation?

Given the following serializer and input data, what will serializer.is_valid() return?

Django

from rest_framework import serializers
from myapp.models import Product

class ProductSerializer(serializers.ModelSerializer):
    class Meta:
        model = Product
        fields = ['id', 'name', 'price']

input_data = {'name': 'Book', 'price': -10}
serializer = ProductSerializer(data=input_data)
valid = serializer.is_valid()

A False, because price cannot be negative

B True, because ModelSerializer does not validate field values by default

C Raises a ValidationError immediately on is_valid() call

D True, but serializer.errors will contain the price error

💡 Hint

Think about how ModelSerializer uses model field validators automatically.

ModelSerializer uses the model's field validators, so negative price (assuming model has positive constraint) causes is_valid() to return False.

📝 Syntax

intermediate

Which option correctly defines a ModelSerializer for a model with a read-only field?

Choose the correct ModelSerializer code that makes the created_at field read-only.

Django

from rest_framework import serializers
from myapp.models import Event

A

class EventSerializer(serializers.ModelSerializer):
    class Meta:
        model = Event
        fields = ['id', 'name']
        read_only_fields = ['created_at']

B

class EventSerializer(serializers.ModelSerializer):
    class Meta:
        model = Event
        fields = ['id', 'name', 'created_at']
        read_only_fields = ['created_at']

C

class EventSerializer(serializers.ModelSerializer):
    created_at = serializers.ReadOnlyField()
    class Meta:
        model = Event
        fields = ['id', 'name', 'created_at']

D

class EventSerializer(serializers.ModelSerializer):
    class Meta:
        model = Event
        fields = ['id', 'name', 'created_at']
    created_at = serializers.ReadOnlyField()

💡 Hint

Check how to specify read-only fields in Meta class.

The read_only_fields attribute inside Meta is the standard way to mark model fields as read-only in ModelSerializer.

🔧 Debug

advanced

Why does this ModelSerializer raise a ValidationError on validation?

Consider this serializer and model. Why does calling serializer.is_valid(raise_exception=True) raise a ValidationError?

Django

class UserSerializer(serializers.ModelSerializer):
    class Meta:
        model = User
        fields = ['username', 'email', 'password']

serializer = UserSerializer(data={'username': 'alice', 'email': '[e-mail]'})
serializer.is_valid(raise_exception=True)
serializer.save()

A Because is_valid() was not called before save()

B Because serializer.save() requires an explicit commit argument

C Because 'password' is missing in input data and is required by the model

D Because the model User does not have an email field

💡 Hint

Check which fields are required and if they are provided.

The serializer expects all fields listed in fields. Missing 'password' causes ValidationError on is_valid().

❓ state_output

advanced

What is the value of serializer.validated_data after is_valid()?

Given this serializer and input, what will serializer.validated_data contain after is_valid()?

Django

class ArticleSerializer(serializers.ModelSerializer):
    class Meta:
        model = Article
        fields = ['title', 'content', 'author']

input_data = {'title': 'Hello', 'content': 'World', 'author': 5}
serializer = ArticleSerializer(data=input_data)
serializer.is_valid()
result = serializer.validated_data

A Empty dict because is_valid() was not called with raise_exception=True

B {'title': 'Hello', 'content': 'World', 'author': <User instance with id=5>}

C Raises ValidationError because author must be a User object

D {'title': 'Hello', 'content': 'World', 'author': 5}

💡 Hint

Recall the default serializer field used for model ForeignKey fields in ModelSerializer.

ModelSerializer uses PrimaryKeyRelatedField by default for foreign key relations, so validated_data contains the primary key value (5), not a model instance.

🧠 Conceptual

expert

Which statement about ModelSerializer behavior is true?

Choose the correct statement about how ModelSerializer handles nested writable relationships.

A You must override <code>create()</code> and <code>update()</code> to handle writable nested serializers

B Nested serializers are always read-only in ModelSerializer

C ModelSerializer automatically supports writable nested serializers without extra code

D ModelSerializer does not support nested serializers at all

💡 Hint

Think about how nested data is saved in DRF.

Writable nested serializers require custom create() and update() methods to save nested data properly.