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')
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')

Add validator that selected father is male and mother is female. Validate that the animal is younger than it's parents.
Add validator that selected father is male and mother is female.
Validate that the animal is younger than it's parents.
