Django rest framework – using SerializerMethodField with ModelSerializer

Question

I have the following models.

models.py

class Language(models.Model):
    name = models.CharField(max_length=255, unique=True)

class Subject(models.Model):
    name = models.CharField(max_length=255, unique=True)

class Term(models.Model):
    subject = models.ForeignKey(Subject, on_delete=models.CASCADE)
    language = models.ForeignKey(Language, on_delete=models.CASCADE)
    name = models.CharField(max_length=255)
    definition = models.TextField()
    image = models.ImageField(default='', blank=True)

I want to write a Serializer that will return a list of subjects. Each subject has a term field, which contains None or a single object, depending on the condition.

serializers.py

class TermSerializer(serializers.ModelSerializer):
    language = serializers.CharField(source='language.name')

    def to_representation(self, data):
        data = data.get(language=self.context['language'])
        return super(TermSerializer, self).to_representation(data)

    class Meta:
        model = Term
        fields = ('id', 'language', 'name', 'definition', 'image')

class FilteredSubjectSerializer(serializers.ListSerializer):
    def to_representation(self, data):
        if self.context['condition']:
            terms = Term.objects.filter(language=self.context['language'])
            data = data.filter(term__in=terms)
        return super(FilteredSubjectSerializer, self).to_representation(data)

class SubjectSerializer(serializers.ModelSerializer):
    term = serializers.SerializerMethodField()

    def get_term(self, term):
        if self.context['condition']:
            return TermSerializer(many=False, source='term_set').data
        return None

    class Meta:
        model = Term
        list_serializer_class = FilteredSubjectSerializer
        fields = ('id', 'name', 'definition', 'image', 'term')

The problem is that when condition == True, the ViewSet returns incorrect data. All fields inside term have a default value.

It works fine if I write SubjectSerializer like this:

class SubjectSerializer(serializers.ModelSerializer):
    term = serializers.TermSerializer(many=False, source='term_set')

    class Meta:
        model = Term
        list_serializer_class = FilteredSubjectSerializer
        fields = ('id', 'name', 'definition', 'image', 'term')

But the case when condition == False doesn’t work.

Asked By: Alexey

||

Source

Answer 1

Try passing the serializer the instance and the context.
Like this:

    def get_term(self, instance):
        if self.context['condition']:
            return TermSerializer(many=False, source='term_set', instance=instance, context=self.context).data
        return None

Answered By: Kristof Rado

Django rest framework – using SerializerMethodField with ModelSerializer

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