uncloud-mravi/uncloud/uncloud_pay/serializers.py

from django.contrib.auth import get_user_model
from rest_framework import serializers
from .models import *

from functools import reduce

class BillSerializer(serializers.ModelSerializer):
    class Meta:
        model = Bill
        fields = ['owner', 'amount', 'due_date', 'creation_date',
                'starting_date', 'ending_date', 'paid']

class PaymentSerializer(serializers.ModelSerializer):
    class Meta:
        model = Payment
        fields = ['owner', 'amount', 'source', 'timestamp']

class PaymentMethodSerializer(serializers.ModelSerializer):
    class Meta:
        model = PaymentMethod
        fields = ['owner', 'primary', 'source', 'description']

class OrderSerializer(serializers.ModelSerializer):
    class Meta:
        model = Order
        fields = '__all__'

class UserSerializer(serializers.ModelSerializer):
    class Meta:
        model = get_user_model()
        fields = ['username', 'email', 'balance']

    # Display current 'balance'
    balance = serializers.SerializerMethodField('get_balance')
    def __sum_balance(self, entries):
        return reduce(lambda acc, entry: acc + entry.amount, entries, 0)

    def get_balance(self, user):
        bills = self.__sum_balance(Bill.objects.filter(owner=user))
        payments = self.__sum_balance(Payment.objects.filter(owner=user))
        balance = payments - bills

        return balance