old_contents: from rest_framework import serializers
  from .models import Animal
  
  class AnimalSerializer(serializers.ModelSerializer):
      class Meta:
          model = Animal
          fields = ('id', 'name', 'dob', 'gender', 'active', 'own', 'father', 'mother')
new_contents: from rest_framework import serializers
  from .models import Animal
  
  class AnimalSerializer(serializers.ModelSerializer):
      class Meta:
          model = Animal
          fields = ('id', 'name', 'dob', 'gender', 
                  'active', 'own', 'father', 'mother')
  
      def validate_father(self, father):
          if (father.gender != Animal.MALE):
              raise serializers.ValidationError('The father has to be male.')
  
      def validate_mother(self, mother):
          if (mother.gender != Animal.FEMALE):
              raise serializers.ValidationError('The mother has to be female.')
  
      def validate_dob(self, dob):
          father_id = self.context['request'].data['father']
          if (father_id):
              father = Animal.objects.get(pk = father_id)
              if (father and father.dob > dob):
                  raise serializers.ValidationError('Animal can\'t be older than it\'s father')
  
          mother_id = self.context['request'].data['mother']
          if (mother_id):
              mother = Animal.objects.get(pk = mother_id)
              if (mother and mother.dob > dob):
                  raise serializers.ValidationError('Animal can\'t be older than it\'s mother')
  
subject: Add validator that selected father is male and mother is female. Validate that the animal is younger than it's parents.