Writable nested serializer in django-rest-framework?

Question

My design is as following about Django ModelSerializer. There are model A and model B. Model B has a foreign key field of Model A. For some reasons, I can not use the primary key directly to serialize Model B. As my thought, what I need is to serialize two other fields(unique together in Model A).

And I see the SlugRelatedField must be used for one slug field. I searched there is a NaturalKeyField can support NaturalKeyField. But it looks like it is superseeded by django-rest-framework. But I checked the django-rest-framework, there is no such field at all. Can anyone help?? What should I do?

The code is as following. Model A

class AssetModel(models.Model):
 org = models.ForeignKey(Org, related_name='models')
 name = models.CharField(max_length=128)
 model_type = models.SmallIntegerField(default = 3,choices = MODEL_TYPE )
 directory = models.CharField(max_length = 128)
 ...
 class Meta:
 unique_together = ('org', 'name',)

Model B

class Dataitem(models.Model):
 mod = models.ForeignKey(AssetModel, related_name='dataitems')
 name = models.CharField(max_length=128)
 data_type = models.SmallIntegerField(default =0,choices = DATAITEM_DATATYPE)
 ...

Serializer of model A

class AssetModelSerializer(serializers.ModelSerializer):
 org = serializers.SlugRelatedField(queryset=Org.objects.all(), slug_field='name')
 class Meta:
 model = AssetModel
 fields = ('org', 'name', 'model_type',..

Serializer of model B

class DataitemSerializer(serializers.ModelSerializer):
 class Meta:
 model = Dataitem
 fields = ('mod', 'name','data_type'...)

The primary key of Model A is just a id Django auto added. When serialize the model B, I need to get the org and name of model A. Both read and write are needed.

• Thomas Liu: github.com/tomchristie/django-serializers
• bakkal: I think the better question would be to post your models and explain what you want to achieve, instead of asking about a solution you thought of

Answer 1

Nested Serializer

You can do something like this, define a serializer for Dataitem that can reuse a serializer of the AssetModel model

class AssetModelSerializer(serializers.ModelSerializer):
 class Meta:
 model = AssetModel
 # Fields org and name of AssetModel will be inlcuded by default
class DataitemSerializer(serializers.ModelSerializer):
 class Meta:
 model = Dataitem
 mod = AssetModelSerializer()
 # This is the Dataitem.mod field
 # which is a FK to AssetModel,
 # Now it'll be serilized using the AssetModelSerializer
 # and include the org and name fields of AssetModelSerializer

I prefer this approach because of the control I get. If you serialize using the above you get a structure like this:

data_item = {'name': ..., 'mod': {'org': ..., 'name': ...}}
 ^
 |___ AssetModel fields

Alternatively you can also use depth = n

You can also use depth = 1 in Dataitem

class DataitemSerializer(serializers.ModelSerializer):
 class Meta:
 model = Dataitem
 depth = 1 # Will include fields from related models
 # e.g. the mod FK to AssetModel

Writable Nested Serializer

Because the behavior of nested creates and updates can be ambiguous, and may require complex dependencies between related models, REST framework 3 requires you to always write these methods explicitly.

We have to implement create/update to make this writable as per DRF's documentation

class DataitemSerializer(serializers.ModelSerializer):
 class Meta:
 model = Dataitem
 # Nested serializer
 mod = AssetModelSerializer()
 # Custom create()
 def create(self, validated_data):
 # First we create 'mod' data for the AssetModel
 mod_data = validated_data.pop('mod')
 asset_model = AssetModel.objects.create(**mod_data)
 # Now we create the Dataitem and set the Dataitem.mod FK
 dataitem = Dataitem.objects.create(mod=asset_model, **validated_data)
 # Return a Dataitem instance
 return dataitem

• Thomas Liu: There are other fields in AssetModel, which I do not want to serialize them. To use nested serializer, do I have to define a new serializer only includes org and name fields?
• bakkal: Yes simply use fields = (...) like you did
• Thomas Liu: thanks a lot. But it looks like the write on the mod fields would fail.
• bakkal: No problem! I'd try get a recent version of DRF and read this about writable nested serializers django-rest-framework.org/api-guide/serializers/…
• bakkal: @ThomasLiu You're welcome. I've added an example so you can relate to your models. Be aware that this example is for 1 nested model, but if AssetModel itself needs a FK key you will have to unpack more data from validated_data and create the object, link the FK etc. I suggest you practice this with a simple 1 nested model first. I Hope this puts you on the right track. Please upvote/accept the answer if it helped you! Thank you!

Answer 2

There seems to be a library that does this drf-writable-nested

it handles the creation and serialisation of these types

• OneToOne (direct/reverse)
• ForeignKey (direct/reverse)
• ManyToMany (direct/reverse excluding m2m relations with through model)
• GenericRelation (this is always only reverse)

• euh: Just got to try it out, is_valid acts funny for now but in general it does work!! And is really easy to try. This answer should be bumped up