Re: Need help with this basic query (annotate vs aggregate)

On 4 Mar, 01:19, Steven Sacks <stevensa...@gmail.com> wrote:
> Here is my model:
>
> class PlaylistTag(models.Model):
>     playlist = models.ForeignKey(Playlist)
>     tag = models.CharField(max_length=128)
>     tag_count = models.PositiveIntegerField(default=1)
>
> The way this model works is simple. If you add a tag to a playlist,
> and the tag doesn't exist, the tag_count is 1. If you add the same tag
> again to that playlist, the tag_count is incremented. If you remove
> that tag, it is decremented. If it reaches 0, I remove the row
> entirely.
>
> A playlist can have multiple tags and a tag can belong to different
> playlists.
>
> (...)
>
> Does this make sense?

IMHO - no, that doesn't make sense :-)

What you need fior the start - and in fact write about - is:

playlists = ManyToManyField(Playlist)

and no `tag_count` field, unless you want denormalize your schema -
which you probably don't.

To count tags for given playlist, you simply do:

playlist.playlisttag_set.count()

Last but not least, looking at your SQL query it might be that I don't
quite get what you need.

--
Tomasz Zielinski
pyconsultant.eu

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