Cannot query "object": Must be "User" instance

Question

I am creating sample-api which have posts and followers. Post should visible to followers only

My models.py

from django.contrib.auth.models import User

class Post(models.Model):
    creator = models.ForeignKey(User, related_name='creator_post_set', null=True, on_delete=models.CASCADE)
    title = models.CharField(max_length=25)
    created_date = models.DateTimeField(auto_now_add=True)
    content = models.TextField()
    likes = models.BigIntegerField(null=True)
    comments = models.BigIntegerField(null=True)

class Follow(models.Model):
    follower = models.ForeignKey(User, related_name='following', null=True, on_delete=models.CASCADE)
    followed_on = models.DateTimeField(auto_now_add=True)
    following = models.ForeignKey(User, related_name='follower',null=True, on_delete=models.CASCADE)

My serializers.py for the models:

class UserSerializer(ModelSerializer):
    password = serializers.CharField(write_only=True)
    def create(self, validated_data):
        user = User.objects.create_user(
            username=validated_data['username'],
            password=validated_data['password'],
            first_name=validated_data['first_name'],
            last_name=validated_data['last_name'],
        )
        return user

    class Meta:
        model = User
        fields = ('password', 'username', 'first_name', 'last_name',)

class PostListSerializer(serializers.ModelSerializer):
    class Meta:
        model = Post
        fields = ['title', 'content', 'created_date',]

 class FollowSerializer(serializers.ModelSerializer):
    class Meta:
        model = Follow
        fields = '__all__'

My views.py:

    class PostList(generics.ListCreateAPIView):
            serializer_class = PostListSerializer
            follow_model = FollowSerializer.Meta.model
            post_model = PostSerializer.Meta.model

            def get_queryset(self):
                try:
                    followers = self.follow_model.objects.get(follower_id = 
 self.request.user.id) 
                    queryset = self.post_model.objects.get(creator__in = followers)
                except self.follow_model.DoesNotExist:
                    queryset = None
                return queryset

When I call this view it returns the following error:

Cannot query "Follow object (1)": Must be "User" instance.

I need help Thanks in Advance.

this isn't the full version of traceback....pls add the whole traceback — JPG
– JPG, Commented Apr 23, 2019 at 2:20
I need the code for your serializers so I can replicate the problem — HuLu ViCa
– HuLu ViCa, Commented Apr 23, 2019 at 2:39

ruddra · Accepted Answer · 2019-04-23 16:36:02Z

2

As I can see, Post model's creator is FKed to User model. So you need to query using User model instance, not Follower model.

You can use the following code:

following = self.request.user.following.all().values_list('follower', flat=True)  # because of related name
queryset = self.post_model.objects.filter(creator_id__in = list(following))

Here I have first retrieved the user ids using self.request.following.all() by reverse relationship. Then I have extracted the user ids using values_list. After that, I have used it in Post.objects.filter(...) method.

edited Apr 23, 2019 at 16:36

answered Apr 23, 2019 at 2:58

ruddra

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3 Comments

Loud JM Over a year ago

I used the above code it returns post Post matching query does not exist.

ruddra Over a year ago

Maybe I queried with wrong values. Can you please try with valurs_list('follower', flat=True)?

Loud JM Over a year ago

thanks @ruddra it works inspite of using get I am using filter(Post.bojects.filter())

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