Django Rest Framework custom validation function on ForeignKey field is not called

Question

I have a ModelSerializer that has a ForeignKey field. This is the code for the model:

class Attendance(models.Model):
    employee = models.ForeignKey(Employee, related_name='attendance_times')
    datetime = models.DateTimeField()

and this is the serializer:

class AttendanceSerializer(serializers.ModelSerializer):

    def validate_employee(self, value):
        try:
            Employee.objects.get(pk=value)
        except Employee.DoesNotExist:
            Employee.objects.create(pk=value)
        return value

    class Meta:
        model = Attendance
        fields = ('employee', 'datetime')

the problem is when I send a post request to create a new object, the function validate_employee is not called and the serializer returns a validation error saying:

Invalid pk "1321" - object does not exist.

Why is this happening? Am I doing something wrong? Is there another validator that is being called before my validator?

Answer 1

There are a couple of things that happen before you the validate_<field> is called.

In particular, the field itself will perform an initial validation check and cast the incoming data into Python object. With PrimaryKeyRelatedField it'll check the associated model and return the instance.

Answer 2

I think you need to call save() function after creating the instance.

    except Employee.DoesNotExist:
        employee = Employee.objects.create(pk=value)
        employee.save()
    return value