serializers.py

class OrganizationContactSerializer(ContactSerializer):
    organization = serializers.PrimaryKeyRelatedField(queryset=Organization.objects.all(), source='content_object')

    class Meta:
        model = Contact
        fields = 'id', 'organization', 'relation', 'type', 'email', 'first_name', 'last_name', 'legal_entity_name', \
                 'postal_code', 'region', 'country', 'role', 'to_user'