Serialize ManyToManyFields with a Through Model in Django REST Framework

Question

I have this M2M relation with through model as

class Person(models.Model):
    name = models.CharField(max_length=128)

    def __str__(self):
        return self.name


class Group(models.Model):
    name = models.CharField(max_length=128)
    members = models.ManyToManyField(Person, through='Membership')

    def __str__(self):
        return self.name


class Membership(models.Model):
    person = models.ForeignKey(Person, on_delete=models.CASCADE)
    group = models.ForeignKey(Group, on_delete=models.CASCADE)
    date_joined = models.DateField()
    invite_reason = models.CharField(max_length=64)

Please note that, I have extra fields date_joined and invite_reason in the through model.

Now, I want to serialize the Group queryset using DRF and thus I choose the below serializer setup.

class PersonSerializer(serializers.ModelSerializer):
    class Meta:
        model = Person
        fields = "__all__"


class GroupSerializer(serializers.ModelSerializer):
    members = PersonSerializer(read_only=True, many=True)

    class Meta:
        model = Group
        fields = "__all__"

and it is returning the following response,

[
    {
        "id": 1,
        "members": [
            {
                "id": 1,
                "name": "Jerin"
            }
        ],
        "name": "Developer"
    },
    {
        "id": 2,
        "members": [
            {
                "id": 1,
                "name": "Jerin"
            }
        ],
        "name": "Team Lead"
    }
]

Here, the members field returning the Person information, which is perfect.

But,

How can I add the date_joined and invite_reason field/info into the members field of the JSON response?

Answer 1

class PersonSerializer(serializers.ModelSerializer):
    class Meta:
        model = Person
        fields = "__all__"

    def serialize_membership(self, person_instance):
        # simple method to serialize the through model fields
        membership_instance = person_instance \
            .membership_set \
            .filter(group=self.context["group_instance"]) \
            .first()

        if membership_instance:
            return MembershipSerializer(membership_instance).data
        return {}

    def to_representation(self, instance):
        rep = super().to_representation(instance)
        return {**rep, **self.serialize_membership(instance)}


class MembershipSerializer(serializers.ModelSerializer): # create new serializer to serialize the through model fields
    class Meta:
        model = Membership
        fields = ("date_joined", "invite_reason")


class GroupSerializer(serializers.ModelSerializer):
    members = serializers.SerializerMethodField() # use `SerializerMethodField`, can be used to pass context data 

    def get_members(self, group):
        return PersonSerializer(
            group.members.all(),
            many=True,
            context={"group_instance": group}  # should pass this `group` instance as context variable for filtering
        ).data

    class Meta:
        model = Group
        fields = "__all__"

Notes:

• Override the to_representation(...) method of PersonSerializer to inject extra data into the members field of the JSON
• We need person instance/pk and group instance/pk to identify the Membership instance to be serialized. For that, we have used the serializer context to pass essential data