Question

The title is probably confusing but I didn’t know how else to paraphrase it. Here is an example.

Is it better to do something like this

class UserType( models.Model ) :
    def user_count( self ) :
        return self.userprofile_set.count()

or like this?

class UserType( models.Model ) :
    def user_count( self ) :
        # Does "user_types = self" work? I'm not sure.
        return UserProfile.filter( user_types = self.pk ).count()

where

class UserProfile( models.Model ) :
    user_types = models.ManyToManyField( UserType )

UserType.user_count() simply returns the number of UserProfile who has a particular UserType.

Sorry if this is a stupid, trivial question, but I was curious. Perhaps some Django expert can dig in on the actual SQL performance or something :-).

Answer 1

They are the same. Choose whatever you like.

In my opinion the first looks cleaner. The second seems a bit incorrect because user_types is not a single pk value.

By the way, it is recommended to always specify a related_name attribute.

For example if it would be:

user_types = models.ManyToManyField(UserType, related_name='profiles')

and then:

return self.profiles.count()

Looks even cleaner.

But both statements generate the same sql, we just could say that first is the shortcut for second. So it’s not about sql-performance