Django Managers

I have the following models code :

from django.db import models
from categories.models import Category

class MusicManager(models.Manager):
    def get_query_set(self):
        return super(MusicManager, self).get_query_set().filter(category='Music')
    def count_music(self):
        return self.all().count()

class SportManager(models.Manager):
    def get_query_set(self):
        return super(MusicManager, self).get_query_set().filter(category='Sport')        

class Event(models.Model): 
    title = models.CharField(max_length=120)
    category = models.ForeignKey(Category)
    objects = models.Manager()
    music = MusicManage开发者_如何转开发r()
    sport = SportManager()

Now by registering MusicManager() and SportManager() I am able to call Event.music.all() and Event.sport.all() queries. But how can I create Event.music.count() ? Should I call self.all() in count_music() function of MusicManager to query only on elements with 'Music' category or do I still need to filter through them in search for category first ?

You can think of a manager as a 'starting point' for a query - you can continue to chain filters just as if you'd started out with the default manager.

For example, Event.objects.filter(category='Music').filter(title='Beatles Concert') is functionally equivalent to Event.music.filter(title='Beatles Concert')

So, as Daniel says, you don't really need to do anything special, just choose one of your custom managers instead of objects and go from there.

You don't need to do anything (and your count_music method is unnecessary). The count() method will use the existing query as defined by get_query_set.