old_contents: from rest_framework import serializers
  
  from users.models import User
  
  class UserSerializer(serializers.ModelSerializer):
  
      class Meta:
          model = User
          fields = ('id', 'full_name', 'is_admin', 'email')
          read_only_fields = ('full_name', 'is_admin', 'email')
  
new_contents: from rest_framework import serializers
  
  from users.models import User
  
  class UserSerializer(serializers.ModelSerializer):
  
      class Meta:
          model = User
          fields = ('id', 'full_name', 'is_admin', 'email', 'username')
          read_only_fields = ('full_name', 'is_admin', 'email', 'username')
  
subject: change fields on serializers