How make QuerySet of items which contain other model.

Question

So i have one model with list of all item i have. And other model which has Foreign-Key to to my main model... How to Make query set object. Of all items from first model which contain my second model?

class Music(models.Model):
    name=models.CharField()
    duration=models.IntegerField()


class Playlist(models.Model):
    misic=models.ForeignKey(Music)

How to Make query set obj of Music which is in Playlist? Then iwant to make .aggregate to count duration of all playlist.

Answer 1

How to Make query set obj of Music which is in Playlist?

Something like this.

playlist = Playlist.objects.get(**conditions)
music = Music.objects.filter(playlist = playlist)

Then iwant to make .aggregate to count duration of all playlist.

Use a Django annotation with the Sum class (django.db.models).

from django.db.models import Sum
qs = Music.objects.filter(playlist = playlist).annotate(total = 
     Sum('duration'))

Update

(After reading @Pol's comment)

Music model has not attribute playlist ...

1. Are you getting an error? If so post the code you used and stack trace. I tested out your models locally and the above given filter/annotation worked fine.
2. Your models don't make sense to me. I'd expect Music and Playlist models to have a many to many relationship. Failing which a Playlist should have many Music instances associated. I.e. Foreign Key should be inside Music, not other way round as you have now.

Update 2

(After reading @Pol's comment)

Yep... You right... It has to be many to many.. but it's not... What should i do? And Music is already made big table in database. I just need to add support of playlists....

There is no need to change your Music model or underlying table. You can add a new model Playlist with the following structure:

class Playlist(models.Model):
    music = models.ManyToManyField(Music)

You can now drop the existing playlist table and execute python manage.py syncdb. The new table for Playlist and join tables for many to many will be created.

That said I must add a statutory warning that this is not a good way of managing changes to your models. You will be much better off using a migration tool such as South. South will let you handle migrations much more gracefully, especially when there is data change involved.