Django Custom Manager add attribute

Question

I have a Django Model that looks like this:

class MyModel(models.Model):
    field1 = models.IntegerField()
    field2 = models.IntegerField()
    nonDbField = SomeObject()
    objects = MyCustomManager()

field1 is actually a PK to an abstract class of SomeObject.

I want a custom manager that for every value returned by any of the functions (all, filter, get, etc) does the following:

 value.nonDbField = SomeObject.objects.get(pk=value.field1)

I've tested that I can manually override get like so:

class MyCustomManager(models.Manager):
    def get(self, *args, **kwargs):
        value = super(MyCustomManager, self).get(*args, **kwargs)
        value.nonDbField = SomeObject.objects.get(listid=value.itemListID)
        return value

but wondered if there was an easier way to do it across all functions.

There's going to be plenty of you that will say, "Why are you doing this?". It has to do with a model inheritance of a legacy, but still active database.

Answer 1

If you need nonDbField's value to be related to the field1 (or any other field in the model) you can try something like this:

Class MyModel(models.Model):

    # your fields here...

    def _nonDbField(self):
        return SomeObject.objects.get(pk=self.field1)
    nonDbField = property(_nonDbField)

This allows you to do something like this:

MyModel.objects.get(pk=1).nonDbField

Keep in mind that you are making a database query each time you access nonDbField (which may or may not be detrimental to your DB performance).