updating multiple database tables in one PUT/PATCH

I'm writing a django-rest-framework backend with rest-framework-mongoengine. So far I have schemas for 2 types - for User and for Device(Box). Sources as follows:

models.py:

from __future__ import unicode_literals

import datetime
from mongoengine import Document, connect, EmbeddedDocument, fields, DynamicDocument
from django.db import models

# Create your models here.
from mongoengine import signals

connect('yourdb', alias='default')

class GPS(EmbeddedDocument):
    lat = fields.FloatField(null=False, required=True)
    lon = fields.FloatField(null=False, required=True)


class PPM(EmbeddedDocument):
    time = fields.DateTimeField(default=datetime.datetime.now())
    value = fields.IntField(null=False, required=True)

    @classmethod
    def pre_save(cls, sender, document, **kwargs):
        document.time = datetime.datetime.now()


signals.pre_save.connect(PPM.pre_save, sender=PPM)

class BuyHistory(EmbeddedDocument):
    time = fields.DateTimeField(default=datetime.datetime.now())
    boxid = fields.StringField(max_length=128, null=False, required=True)
    username = fields.StringField(max_length=128, null=False, required=True)
    product = fields.StringField(max_length=128, null=False, required=True)
    amount = fields.IntField()

    @classmethod
    def pre_save(cls, sender, document, **kwargs):
        document.time = datetime.datetime.now()


signals.pre_save.connect(BuyHistory.pre_save, sender=BuyHistory)

class RecycleHistory(EmbeddedDocument):
    time = fields.DateTimeField(default=datetime.datetime.now())
    boxid = fields.StringField(max_length=128, null=False, required=True)
    username = fields.StringField(max_length=128, null=False, required=True)
    amount = fields.IntField()

    @classmethod
    def pre_save(cls, sender, document, **kwargs):
        document.time = datetime.datetime.now()


signals.pre_save.connect(RecycleHistory.pre_save, sender=RecycleHistory)

class Box(Document):
    boxid = fields.StringField(max_length=128, null=False, required=True)
    gps = fields.EmbeddedDocumentField(GPS, required=True)
    buy_history = fields.EmbeddedDocumentListField(BuyHistory, default='[]')
    recycle_history = fields.EmbeddedDocumentListField(RecycleHistory, default='[]')
    ppm_history = fields.EmbeddedDocumentListField(PPM, default='[]')

class User(Document):
    username = fields.StringField(max_length=128, null=False, required=True)
    rfid = fields.StringField(max_length=32, null=False, required=True)
    buy_history = fields.EmbeddedDocumentListField(BuyHistory)
    recycle_history = fields.EmbeddedDocumentListField(RecycleHistory)

serializers.py:

from rest_framework_mongoengine import serializers
from models import User, BuyHistory, Box, RecycleHistory, PPM

class UserSerializer(serializers.DocumentSerializer):
    class Meta:
        model = User
        fields = ('username', 'rfid', 'buy_history', 'recycle_history')


class PPMSerializer(serializers.DocumentSerializer):
    class Meta:
        model = PPM
        fields = ('time', 'value')


class BuyHistorySerializer(serializers.EmbeddedDocumentSerializer):
    class Meta:
        model = BuyHistory
        fields = ('time', 'boxid', 'username', 'product', 'amount')


class RecycleHistorySerializer(serializers.EmbeddedDocumentSerializer):
    class Meta:
        model = RecycleHistory
        fields = ('time', 'boxid', 'username', 'product', 'amount')


class BoxSerializer(serializers.DocumentSerializer):
    class Meta:
        model = Box
        fields = ('id', 'boxid', 'gps', 'buy_history', 'recycle_history', 'ppm_history')

    def update(self, instance, validated_data):
        buy = validated_data.pop('buy_history')
        recycle = validated_data.pop('recycle_history')
        ppm = validated_data.pop('ppm_history')
        updated_instance = super(BoxSerializer, self).update(instance, validated_data)

        for buy_data in buy:
            updated_instance.buy_history.append(BuyHistory(**buy_data))
        for recycle_data in recycle:
            updated_instance.recycle_history.append(RecycleHistory(**recycle_data))
        for ppm_data in ppm:
            updated_instance.ppm_history.append(PPM(**ppm_data))
        updated_instance.save()
        return updated_instance

My goal is to update user's buy_history and recycle_history when updating the Box object. How can I do it?

Ok, I have found a solution, apparently it was super simple but if someone has similar confusion here's the answer:

Just get the user with username which is set in nested object like this.

for buy_data in buy:
    buy_history_s = BuyHistory(**buy_data)
    try:
        user = User.objects.get(username=buy_history_s.username)
    except errors.DoesNotExist:
        raise exceptions.AuthenticationFailed()
    if buy_history_s.time is None:
        buy_history_s.time = datetime.datetime.now()
    updated_instance.buy_history.append(buy_history_s)
    user.buy_history.append(buy_history_s)
    user.save()

Cheers