property decorator eager load and eager load in general using Django

Question

Is it possible in Django to use eager loading for property decorator?

My code as example

class Player(models.Model):
    roles = models.ManyToManyField(Role, related_name='players')

    @property
    def role(self):
         return ", ".join([r.name for r in self.roles.all().order_by('name')])

When outputting the player role using the property, it runs a query each time.

Actually I still don't know how eager load works in general with Django. I can't find any good docs about it.

Thanks

possible duplicate of [do properties work on django model fields?](http://stackoverflow.com/questions/1454727/do-properties-work-on-django-model-fields) — Henrik Andersson, Aug 04 '14 at 22:57

score 8 · Accepted Answer · answered Aug 07 '14 at 01:43

8

This is a terminology issue. Django docs/source do not refer to "eager" loading as SQLAlchemy does. Instead, Django has select_related() and prefetch_related() filters for queries, which are documented in the QuerySet API reference.

But that is something you add to the query, not a declarative property. So you'd use prefetch_related('roles') when querying for the Player.

answered Aug 07 '14 at 01:43

Jason S

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what if I need to preload a field on a Join Table? – Pietro Aug 17 '14 at 06:35

score 1 · Answer 2 · answered Aug 06 '14 at 17:46

I'm not sure if this fits your needs, because this is useful for when you want to use the property of an instance several times, but Django has a @cached_property decorator which will cache the result of your property for as long as your instance exists.

More information if you do a quick ctrl+f in here:

https://docs.djangoproject.com/en/dev/ref/utils/

property decorator eager load and eager load in general using Django

2 Answers2