Django serializer - unable to pass correct data in

Question

I am trying to solve this problem :

A user can add skills to his profile and other user can upvote the skill on his profile .

I have implemented adding skills in system . Now next I am trying build adding a skill (which is already added in system by admins ) to be added to the user profile .

But in my POST API , i am always getting following error

{
    "user": {
        "user": [
            "This field is required."
        ]
    }
}

Body Input :

{
    "user":{
         "username": "USERN",
         "email": "[e-mail]",
         "first_name": "",
          "last_name": ""
    },
    "new_user_skill":
        {
             "id": 1,
             "skill_name": "C"
        }
}

My View :

 elif request.method == 'POST':
        data = { 'user':request.data.get('user'),'skill_item':request.data.get('new_user_skill')}
        serializer = UserSkillSerializer(data=data)
        print("-------------------> serializer ")
        print(serializer)
        if serializer.is_valid():
            serializer.save()
            return Response(serializer.data, status=status.HTTP_201_CREATED)
        return Response(serializer.errors, status=status.HTTP_400_BAD_REQUEST)  

Model :

class UserModelSerializer(serializers.ModelSerializer):
    class Meta:
        model = User
        fields = ('username', 'first_name', 'last_name', 'email')
        extra_kwargs = {
            'username': {
                'validators': [],
            }
        }    


class UserProfileSerializer(serializers.ModelSerializer):
    user=UserModelSerializer()
    class Meta:
        model = UserProfile
        fields = '__all__'

        def create(self, validated_data):

            user_serializer = UserModelSerializer.create(UserModelSerializer(),validated_data = validated_data)
            user,created=UserProfile.objects.update_or_create(user=user_serializer)
            return user

class UserSkillSerializer(serializers.ModelSerializer):

    user = UserProfileSerializer(required=True)
    skill_item = SkillSerializer(required=True)
    class Meta:
        model = UserSkill
        fields= '__all__'

    def create (self,validated_data):

        user_data = validated_data.pop('user')
        user = UserProfileSerializer.create(UserProfileSerializer(),validated_data= user_data)
        skill_data = validated_data.pop('skill_item')
        skill_item   = SkillSerializer.create(SkillSerializer(),validated_data=skill_data)
        user_skill, created = UserSkill.objects.update_or_create(user=user,skill_item=skill_item)

        return user_skill

Serializer :

class SkillSerializer(serializers.ModelSerializer):

    class Meta:
        model = Skill
        fields = '__all__'
        extra_kwargs = {
            'skill_name': {
                'validators': [],
            }
        }  

class UserModelSerializer(serializers.ModelSerializer):
    class Meta:
        model = User
        fields = ('username', 'first_name', 'last_name', 'email')
        extra_kwargs = {
            'username': {
                'validators': [],
            }
        }    


class UserProfileSerializer(serializers.ModelSerializer):
    user=UserModelSerializer()
    class Meta:
        model = UserProfile
        fields = '__all__'

        def create(self, validated_data):

            user_serializer = UserModelSerializer.create(UserModelSerializer(),validated_data = validated_data)
            user,created=UserProfile.objects.update_or_create(user=user_serializer)
            return user

class UserSkillSerializer(serializers.ModelSerializer):

    user = UserProfileSerializer(required=True)
    skill_item = SkillSerializer(required=True)
    class Meta:
        model = UserSkill
        fields= '__all__'

    def create (self,validated_data):

        user_data = validated_data.pop('user')
        user = UserProfileSerializer.create(UserProfileSerializer(),validated_data= user_data)
        skill_data = validated_data.pop('skill_item')
        skill_item   = SkillSerializer.create(SkillSerializer(),validated_data=skill_data)
        user_skill, created = UserSkill.objects.update_or_create(user=user,skill_item=skill_item)

        return user_skill

I have removed validators (not sure if that is correct approach )

Edit 1

I have edited much of my code as per answer (i have studied and implemented reverse relation ) , facing one Not Null constraint even though I have null=True

My Updated code :

Serializer :

class UserSerializer(serializers.ModelSerializer):
    class Meta:
        model = User
        fields = ('username', 'first_name', 'last_name', 'email','user_profile')


class UserSkillSerializer(serializers.ModelSerializer):

    user = serializers.PrimaryKeyRelatedField(read_only=True)
    skill_item = serializers.PrimaryKeyRelatedField(read_only=True)
    class Meta:
        model = UserSkill
        fields= ('user', 'skill_item')

Views :

elif request.method == 'POST':

    current_user = User.objects.get(pk=request.data.get('user')) # get the user key
    user_profile_id = current_user.user_profile.pk # get the user profile from reverse relation in model
    data = { 'user':user_profile_id,'skill_item':request.data.get('skill_id')}
    serializer = UserSkillSerializer(data=data)
    if serializer.is_valid():
        serializer.save()
        return Response(serializer.data, status=status.HTTP_201_CREATED)
    return Response(serializer.errors, status=status.HTTP_400_BAD_REQUEST)            

Model :

# This class will more or less map to a table in the database and defines the many to many relationship between user-skill, this is our intermediate model
class UserSkill(models.Model):
    """ A Model for representing skill in user profile """
    unique_together = (('user', 'skill_item'),)

    user = models.ForeignKey('UserProfile',on_delete=models.CASCADE,related_name='current_user_skills')

    skill_item = models.ForeignKey(Skill,on_delete=models.CASCADE,null=True)

    def __str__(self):
        """Return a human readable representation of the model instance."""
        return "{}".format(self.skill_item.skill_name)

# this class adds a Many to Many field in existing django-rest auth UserProfile class for  user and his/her skills 
class UserProfile(models.Model):
    user = models.OneToOneField('auth.User',unique=True,on_delete=models.CASCADE,related_name='user_profile')
    user_skills = models.ManyToManyField(
            Skill,
            through='UserSkill',
            through_fields=('user','skill_item'),null=True
        )
#create user profile signal handler
def create_user_profile(sender, instance, created, **kwargs):
    if created:
        UserProfile.objects.create(user=instance)

post_save.connect(create_user_profile, sender=User)

Error :

return func(*args, **kwargs)   File "C:\code\django\wantedly\src\wantedly_webapp\views\AllViews.py", line

61, in user_skill_collection serializer.save()

IntegrityError at /api/v1/user/skills/ NOT NULL constraint failed: wantedly_webapp_userskill.skill_item_id

Answer 1

This error is generated because the UserProfileSerializer has a user field that expects a whole user definition. In general I believe you mix the concepts "user profile" / "user" in your serializers, which creates confusion.

Once you have created a User with its associated UserProfile and a Skill, a way to add the Skill to the UserProfile is by selecting them by primary key:

class UserSkillSerializer(serializers.ModelSerializer):

    user = serializers.PrimaryKeyRelatedField()
    skill_item = serializers.PrimaryKeyRelatedField()

    class Meta:
        model = UserSkill
        fields= ('user', 'skill_item')

The POST data to assign a Skill to a UserProfile would then be:

{
    "user": 3,  # PK of the UserProfile, not the User!
    "skill_item": 42  # PK of the SkillItem you want to assign
}