ManyToManyField Serializer throws "This field must be unique" error - django

Question

I am trying to create a Many-To-Many relationship between two models- Author and Book. My use-case is that I should be able to add a new book to the database with an author that already exists in the database.
models.py
class Author(models.Model):
author_id = models.CharField(max_length=20, primary_key=True)
name = models.CharField(blank=True, null=True)
def __unicode__(self):
return self.name
class Meta:
ordering = ('author_id',)
class Book(models.Model):
title = models.CharField(max_length=50, primary_key=True)
authors = models.ManyToManyField(Author)
def __unicode__(self):
return self.title
class Meta:
ordering = ('title',)
serializers.py
class AuthorSerializer(serializers.ModelSerializer):
class Meta:
model = Author
fields = ('author_id', 'name')
class BookSerializer(serializers.ModelSerializer):
authors = AuthorSerializer(many=True)
class Meta:
model = Book
fields = ('title', 'authors')
def create(self, validated_data):
book = Book.objects.create(name=validated_data['title'])
for item in validated_data['authors']:
author = Author.objects.get(author_id=item['author_id'])
book.authors.add(author)
return book
Let's say my Author table already has an Author:
1, George RR Martin
Now if I want to add a new book with an existing author, this is the request I send using httpie:
http -j POST http://localhost/books title="The Winds of Winter" authors:='[{"author_id":"1"}]'
and when I do, I get this error:
Output Error
{
"authors": [
{
"author_id": [
"This field must be unique."
]
}
]
}
It seems like the AuthorSerializer is being called which checks the provided author_id against the ones in the database already and throws this error.
Any help on this would be appreciated.

Answer 1

Is there a specific reason you have to use a custom PK field?
Django automatically creates primary key fields for you. If you simply delete that field from your model and your serializer (and create/run a migration on your database), you won't have to specify the pk in your POST call from your frontend, and Django will create an AutoField that auto-increments your model's id:
class Author(models.Model):
# Remove this line and run makemigrations.
# author_id = models.CharField(max_length=20, primary_key=True)
name = models.CharField(blank=True, null=True)
def __unicode__(self):
return self.name
class Meta:
ordering = ('author_id',)
If not, consider using an models.AutoField rather than models.CharField for your primary key field, and again, don't include this in your POST call.
Note, that if you already have a big database created, you might have to do some intricate work in your migration, a la this answer: