DRF: problem with writing nested serializer with unique field - django-rest-framework

Question

I need to add some tags to a post when creating it. They have a many to many relationship, and tag has a unique name field. But I get an already exists error.
Here is my setup:
class Tag(models.Model):
name = models.SlugField(max_length=100, unique=True)
class Post(models.Model):
(...)
tags = models.ManyToManyField(Tag, related_name='posts')
class PostSerializer(serializers.HyperlinkedModelSerializer):
tags = TagSerializer(many=True)
def create(self, validated_data):
tags_data = validated_data.pop('tags')
post = Post.objects.create(**validated_data)
for tag_data in tags_data:
try:
tag = Tag.objects.get(name=tag_data['name'])
except Tag.DoesNotExist:
tag = Tag.objects.create(**tag_data)
post.tags.add(tag)
return post
class Meta:
model = Post
(...)
Now when I post the following data to create a Post:
{
(...),
"tags": [{"name": "someExistentTag"}, {"name": "someTag"}]
}
serializer.is_valid is called prior to create and I get the following response:
{
"tags": [
{
"name": [
"tag with this name already exists."
]
},
{}
]
}
What is your solution?

Answer 1

Here is the first thing I got working; get tags away from post and validate them manually (which I'm not sure is done right). Yet I'd like to see a better solution.
class PostSerializer(HyperlinkedModelSerializer):
tags = TagSerializer(many=True)
def __init__(self, instance=None, data=empty, **kwargs):
super().__init__(instance, data, **kwargs)
if hasattr(self, 'initial_data'):
self.tags = self.initial_data.get('tags', [])
if 'tags' in self.initial_data:
self.initial_data['tags'] = []
def create(self, validated_data):
tags_data = self.tags
existing_tags = []
new_tags_data = []
for tag_data in tags_data:
try:
tag = Tag.objects.get(name=tag_data['name'])
except KeyError:
raise ValidationError("Field 'name' for tag is required.")
except Tag.DoesNotExist:
new_tags_data.append(tag_data)
else:
existing_tags.append(tag)
new_tags_serializer = TagSerializer(data=new_tags_data, many=True)
new_tags_serializer.is_valid(raise_exception=True)
validated_data.pop('tags')
post = Post.objects.create(**validated_data)
for tag_data in new_tags_data:
tag = Tag.objects.create(**tag_data)
post.tags.add(tag)
for tag in existing_tags:
post.tags.add(tag)
return post