AttributeError: Serializer object has no attribute 'initial_data', when trying to post a new request to a nested serializer in DjangoRestFramework

I'm trying to create two model instances from a single post request. Model2 is linked to Model1 using a ForeignKey field. Model1 also has a validate() in its serializer. The issue is when I use nested serializer to save both model instances together, the validate function of Model1 returns an error AttributeError: 'Model1Serializer' object has no attribute 'initial_data'.

As per DRF docs

If the data keyword argument is not passed then the .initial_data attribute will not exist.

Trials: I tried intercepting the flow in __init__() and validate() of CombinedSerializer but it seems, the validate() of Model1Serializer is evaluated first.

Doubt: Why is the Model1Serializer not getting supplied with the data keyword argument and how can I correct my code? At what stage is the Model1Serializer validate() function evaluated?

Models:

from django.db import models

class Model1(models.Model):
    key1 = models.PositiveIntegerField(blank=False, null=False)

    def __str__(self):
        return self.key1

class Model2(models.Model):
    foreign_key = models.ForeignKey('Model1', blank=True, null=True, on_delete=models.CASCADE)
    key2 = models.CharField(max_length=20)

    def __str__(self):
        return self.key2

Serializers:

from rest_framework import serializers
from .models import Model1, Model2

class Model1Serializer(serializers.ModelSerializer):
    def validate(self, data):
         if(self.initial_data['key1']>=10):
            data['key1'] = self.initial_data['key1'] + 1
            return data
         else:
            data['key1'] = self.initial_data['key1']
            return data
      
    class Meta:
        model = Model1
        fields = '__all__'

class Model2Serializer(serializers.ModelSerializer):
    class Meta:
        model = Model2
        fields = '__all__'

class CombinedModelSerializer(serializers.ModelSerializer):
    model1 = Model1Serializer()
    model2 = Model2Serializer()

    def create(self, validated_data):
        model2_data = validated_data.pop('model2')
        model1 = Model1.objects.create(**validated_data)
        model2 = Model2.objects.create(foreign_key=model1.id, **model2_data)
        return model1

    class Meta:
        model = Model1
        fields = '__all__'
    

Views:

from django.shortcuts import render
from rest_framework import generics
from .models import Model1, Model2
from .serializers import Model1Serializer, Model2Serializer, CombinedModelSerializer

class Model1List(generics.ListCreateAPIView):
    queryset = Model1.objects.all()
    serializer_class = Model1Serializer

class Model2List(generics.ListCreateAPIView):
    queryset = Model2.objects.all()
    serializer_class = Model2Serializer

class CombinedList(generics.ListCreateAPIView):
    queryset = Model1.objects.all()
    serializer_class = CombinedModelSerializer

Post Request:

POST: http://127.0.0.1:8000/combined/

{
    "model1":
    {
        "key1": 45
    },
    "model2": {
        "key2": "val2"
    }
}

Package Versions:

Python==3.6.8
Django==2.2.4
djangorestframework==3.10.3