Here's an example of SerializerMethodField from the Django Rest Framework docs:

from django.contrib.auth.models import User
from django.utils.timezone import now
from rest_framework import serializers

class UserSerializer(serializers.ModelSerializer):
    days_since_joined = serializers.SerializerMethodField()

    class Meta:
        model = User

    def get_days_since_joined(self, obj):
        return (now() - obj.date_joined).days

If this get_ stuff hurts your soul like it does to mine, I have a cleaner alternative for you. First, let's create a new Serializer Field.

class SerializerLambdaField(SerializerMethodField):

    def bind(self, field_name, parent):
        super(SerializerMethodField, self).bind(field_name, parent)

    def to_representation(self, data):
        return self.method_name(data)

Now you can write the previous serializer like this:

class UserSerializer(serializers.ModelSerializer):
    days_since_joined = SerializerLambdaField(lambda obj: (now() - obj.date_joined).days)

    class Meta:
        model = User