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Django REST framework: get field of related model in serializer

I'm new to Django Rest Framework. I'm trying to get my ListAPI to show various fields of my Quiz (and related) models. It's working fine, except for my attempt_number field. I'm getting the right queryset, but I'm not sure how to get only the relevant value for every query. Users can take every quiz as many times as they want, and I want to show the queryset for each attempt, since the score etc. will be different.
My model setup is as follows:
class Quiz(models.Model):
title = models.CharField(max_length=15)
slug = models.SlugField(blank=True)
questions_count = models.IntegerField(default=0)
class Question(models.Model):
quiz = models.ForeignKey(Quiz, on_delete=models.CASCADE)
label = models.CharField(max_length=1000)
class Choice(models.Model):
question = models.ForeignKey(Question, on_delete=models.CASCADE)
answer = models.CharField(max_length=100)
is_correct = models.BooleanField('Correct answer', default=False)
class QuizTaker(models.Model):
user = models.ForeignKey(User, on_delete=models.CASCADE)
quiz = models.ForeignKey(Quiz, on_delete=models.CASCADE)
correct_answers = models.IntegerField(default=0)
completed = models.BooleanField(default=False)
attempt_number = models.PositiveIntegerField(default=0)
My serializer for the ListAPI looks as follows:
class MyQuizListSerializer(serializers.ModelSerializer):
attempt = serializers.SerializerMethodField()
# etc..
class Meta:
model = Quiz
fields = "__all__"
def get_attempt(self, obj):
try:
quiztaker = QuizTaker.objects.filter(user=self.context['request'].user,
quiz=obj)
for attempt in quiztaker:
attempt_number = attempt.attempt_number
return attempt_number
If I do it like this, I always get the last value for attempt_number (because the loop overwrites the value). So then I tried to append it to a list instead, like this:
a = []
for attempt in quiztaker:
attempt_number = attempt.attempt_number
a.append(attempt_number)
return a
But then I get the list of attempts for every query, instead of the attempt number for each query. I.e. I get the following three times (because in this case there are three attempts):
{
"id": 4,
"attempt": [
1,
2,
3
]
},
But instead what I want is (and the same for attempt 2 and 3 etc.):
{
"id": 4,
"attempt": 1
},
So I tried doing it like this:
return a[attempt_number-1]
Hoping it would give me index zero for attempt number 1, 1 for 2, etc. But then I still just get the last attempt number (3 in this case). How can I solve this?
I also tried just using an IntegerField instead of a SerializerMethodField as follows:
attempt = serializers.IntegerField(read_only=True, source='quiztaker.attempt_number')
But it returned nothing.

If I correctly understood you, you want the list of attempts added to each quiz object.
{
"id": 4,
"attempts": [{
"id": 1,
"attempt_number": 1,
},
{
"id": 2,
"attempt_number": 2,
}...]
}
In that case, you should have a separate serializer for the QuizTaker model and serialize the objects in the SerializerMethodField.
class QuizTakerSerializer(serializers.ModelSerializer):
class Meta:
model = QuizTaker
fields = ('id', 'attempt_number')
class MyQuizListSerializer(serializers.ModelSerializer):
attempts = serializers.SerializerMethodField()
# etc..
class Meta:
model = Quiz
fields = "__all__"
def get_attempts(self, obj):
quiztakers = QuizTaker.objects.filter(user=self.context['request'].user,quiz=obj)
return QuizTakerSerializer(quiztakers, many=True).data
Honestly, your question is not very clear and it would help to edit it and make it clearer, giving the JSON structure you want to achieve. I also suspect your intended use of queryset isn't the actual Django meaning for a container of ORM objects.

How to get model data to appear as a field in another model's response

These are simplified versions of my models (the user model is just an id and name)
class Convo(models.Model):
owner = models.ForeignKey(User, on_delete=models.CASCADE, related_name='convo_owner')
users = models.ManyToManyField(User, through='Convo_user')
class Convo_user (models.Model):
user = models.ForeignKey(UserProfile, on_delete=models.CASCADE)
convo = models.ForeignKey(Convo, on_delete=models.CASCADE)
class Comments(models.Model):
name = models.CharField(max_length=255)
content = models.TextField(max_length=1024)
convo = models.ForeignKey(Convo, on_delete=models.CASCADE)
This is my view
class ConvoViewSet(viewsets.ModelViewSet):
serializer_class = serializers.ConvoSerializer
def get_queryset(self):
return None
def list(self, request):
curr_user = request.user.id
# Collecting the list of conversations
conversations = models.Conversation.object.filter(ConvoUser__user_id=request.user.id)
#Getting list of conversation id's
conv_ids = list(conversations.values_list('id', flat=True).order_by('id'))
#Getting list of relevant comments
comments = models.Comments.objects.filter(conversation_id__in=conv_ids)
return Response(self.get_serializer(conversations, many=True).data)
And my current serializer
class ConvoSerializer(serializers.ModelSerializer):
"""A serializer for messaging objects"""
# access = AccessSerializer(many=True)
# model = models.Comments
# fields = ('id', 'name', 'content', 'convo_id')
class Meta:
model = models.Convo
fields = ('id', 'owner_id')
The current response I get is of the form
[
{
"id": 1,
"owner_id": 32
}, ...
]
But I would like to add a comments field that shows all the properties of comments into the response, so basically everything in the second queryset (called comments) and I'm not sure how to go about this at all. (I retrieve the comments in the way I do because I'm trying to minimize the calls to the database). Would I need to create a new view for comments, make its own serializer and then somehow combine them into the serializer for the convo?

The way you've set up your models, you can access the comments of each Convo through Django's ORM by using convo_object.comments_set.all(), so you could set up your ConvoSerializer to access that instance's comments, like this:
class ConvoSerializer(serializers.ModelSerializer):
"""A serializer for messaging objects"""
comments_set = CommentSerializer(many=True)
class Meta:
model = models.Convo
fields = ('id', 'owner_id', 'comments_set')
and then you define your CommentSerializer like:
class CommentSerializer(serializers.ModelSerializer):
class Meta:
model = models.Comments
fields = ('id', 'name', 'content')

No data appears because my serializers are using the default database, not sure why but a step forward
EDIT:
Django: Database used for prefetch_related is not the same that the parent query Provided me the correct answer, I was able to choose the database with this method because for some reason inner queries use the default DB

How to efficiently hit the database for a related field in REST serializer?

I have a complex architecture for my database and I'm having troubles writting the corresponding REST API.
Context: I am requesting various informations on a public API every hour, including data I want to keep history for.
Here's the models I am using
class Player(models.Model):
name = models.CharField(max_length=255)
class PlayerStatsHistory(models.Model):
player = models.ForeignKey('Player', null=True, on_delete=models.CASCADE)
last_refresh = models.DateTimeField(null=True)
...
This way, I can store every change in the stats of each user.
I wrote 2 serializers one for Player and one for PlayerStatsHistory. The easiest one works fine
class PlayerStatsSerializer(HyperlinkedModelSerializer):
class Meta:
model = PlayerStatsHistory
fields = ('last_refresh', ...)
But when I need to request the latest stats for one player I'm getting confused :
class PlayerSerializer(HyperlinkedModelSerializer):
details = SerializerMethodField()
def get_details(self, obj):
return PlayerStatsSerializer(PlayerStatsHistory.objects.filter(player=obj).order_by('-last_refresh').first()).data
class Meta:
model = Player
fields = ('name', 'details')
This works fine, but will hit database at each player provided to the PlayerSerializer and I feel like I'm doing it wrong.
How can I improve this solution ?

I think the best you can do is first get the id of the last PlayerStatsHistory of each Player(using group_by):
latest_stats_history_pks = PlayerStatsHistory.objects.values('player').annotate(max_id=models.Max('id')).values_list('max_id', flat=True)
(it has a problem, it uses all of the players if you are using pagination you don't need all of them, in that case, prefetch without filtering its queryset should be fine)
then only prefetch this values on your Player queryset, so:
queryset = Player.objects.all().prefetch_related(models.Prefetch(
'playerstatshistory_set',
queryset=PlayerStatsHistory.objects.filter(pk__in=latest_stats_history_pks), to_attr='last_stat_list'))
so finally your get_queryset method in your view should be like:
def get_queryset(self):
latest_stats_history_pks = PlayerStatsHistory.objects.values('player').annotate(max_id=models.Max('id')).values_list('max_id', flat=True)
queryset = Player.objects.all().prefetch_related(models.Prefetch(
'playerstatshistory_set',
queryset=PlayerStatsHistory.objects.filter(pk__in=latest_stats_history_pks), to_attr='last_stat_list'))
return queryset
and if you're using FBV, do something like this:
from rest_framework.decorators import api_view
from rest_framework.response import Response
#api_view(['GET'])
def player_list(request, format=None):
if request.method == 'GET':
latest_stats_history_pks = PlayerStatsHistory.objects.values('player').annotate(max_id=models.Max('id')).values_list('max_id', flat=True)
players = Player.objects.all().prefetch_related(models.Prefetch(
'playerstatshistory_set',
queryset=PlayerStatsHistory.objects.filter(pk__in=latest_stats_history_pks), to_attr='last_stat_list'))
serializer = PlayerSerializer(players, many=True)
return Response(serializer.data)
also in your serializer, change that field as below:
class PlayerSerializer(HyperlinkedModelSerializer):
details = SerializerMethodField()
def get_details(self, obj):
return {} if not obj.last_stat_list else PlayerStatsSerializer(obj.last_stat_list[-1]).data
class Meta:
model = Player
fields = ('name', 'details')

Django Twitter clone. How to restrict user from liking a tweet more than once?

I'm not sure where to start. Right now, the user can press like as many times they want and it'll just add up the total likes for that tweet.
models.py
class Howl(models.Model):
author = models.ForeignKey(User, null=True)
content = models.CharField(max_length=150)
published_date = models.DateTimeField(default=timezone.now)
like_count = models.IntegerField(default=0)
rehowl_count = models.IntegerField(default=0)
def get_absolute_url(self):
return reverse('howl:index')
def __str__(self):
return self.content
views.py
class HowlLike(UpdateView):
model = Howl
fields = []
def form_valid(self, form):
instance = form.save(commit=False)
instance.like_count += 1
instance.save()
return redirect('howl:index')
Django Twitter clone. How to restrict user from liking a tweet more than once?

As well as tracking how many Likes a post has, you'll probably also want to track who has "Liked" each post. You can solve both of these problems by creating a joining table Likes with a unique key on User and Howl.
The unique key will prevent any User from doing duplicate likes.
You can do this in Django with a ManyToManyField, note that since this means adding a second User relationship to Howl, we need to disambiguate the relationship by providing a related_name
Eg:
class Howl(models.Model):
author = models.ForeignKey(User, null=True, related_name='howls_authored')
liked_by = models.ManyToManyField(User, through='Like')
# ...rest of class as above
class Like(models.Model):
user = models.ForeignKey(User)
howl = models.ForeignKey(Howl)
class Meta:
unique_together = (('user', 'howl'))
like_count count then becomes redundant, since you can use Howl.liked_by.count() instead.
The other benefit of this is that it allows you to store information about the Like - eg when it was added.

An idea could be adding a column to your table named likers and before incrementing like_counts check if the models.likers contains the new liker or not. If not increment the likes, if yes don't.

Changed liked_count in my models.py to
liked_by = models.ManyToManyField(User, related_name="likes")
views.py
class HowlLike(UpdateView):
model = Howl
fields = []
def form_valid(self, form):
instance = form.save(commit=False)
instance.liked_by.add(self.request.user)
instance.like_count = instance.liked_by.count()
instance.save()
return redirect('howl:index')
index.html
{{howl.liked_by.count}}

Django Rest Framework: dynamic database on POST - RelatedField or PrimaryKeyRelatedField

I'm developing RESTFul services with DRF and I have multiple databases depending on the country (see my last question here)
I'm having a problem now with relationships, I have two models: Category and SubCategory:
class SubCategory(models.Model):
objects = CountryQuerySet.as_manager()
id = models.AutoField(primary_key=True,db_column='sub_category_id')
name = models.TextField()
female_items_in_category = models.BooleanField()
male_items_in_category = models.BooleanField()
kids_items_in_category = models.BooleanField()
category = models.ForeignKey('Category')
class Meta:
managed = True
db_table = Constants().SUBCATEGORY
And the serializer is:
class SubCategorySerializer(serializers.ModelSerializer):
category = PrimaryKeyRelatedField(queryset=Category.objects.using('es').all())
class Meta:
model = SubCategory
fields = ('id', 'name','female_items_in_category','male_items_in_category','kids_items_in_category','category')
If I don't set the queryset with the proper country it fails, because it doesn't know where to get the category.
Here the problem
I already set the country in the serializer context (in the ModelViewSet):
def get_serializer_context(self):
return {Constants().COUNTRY: self.kwargs.get(Constants().COUNTRY)}
But I can not find the proper way to get the self.context.get(Constants().COUNTRY) in the serializer.
Do you any have an idea to solve this? Thanks!

Well, I found a solution to my problem: I overwrite the method get_fields in the serializer:
def get_fields(self, *args, **kwargs):
fields = super(SubCategorySerializer, self).get_fields()
country = self.context.get(Constants().COUNTRY)
qs = Category.objects.using(country).all()
fields['category'].queryset = qs
return fields
And that works!