On Saturday, 10 September 2011 05:46:33 UTC+1, Danny wrote:
>
> Hi, I'm learning Django (albeit slowly) and I'm trying to set up a really
> simple database. I want to have Artists, Albums, and Tracks. I want to be
> able to navigate both ways in the db. So you should be able to go from
> artist -> album or from album -> artist. Same thing with album and track. I
> really can't figure it out. Here's the simplest test case I can come up
> with:
>
> class Song(models.Model):
> album = models.ForeignKey('Album')
>
> class Album(models.Model):
> tracks = models.ManyToManyField(Song)
>
> when i run syncdb, it comes up with the following error:
> music.album: Reverse query name for m2m field 'tracks' clashes with field
> 'Song.album'. Add a related_name argument to the definition for 'tracks'.
>
> So I go back and add the related_name argument:
> class Song(models.Model):
> album = models.ForeignKey('Album')
>
> class Album(models.Model):
> tracks = models.ManyToManyField(Song, related_name='album')
>
> and run syncdb again:
> music.album: Accessor for m2m field 'tracks' clashes with field
> 'Song.album'. Add a related_name argument to the definition for 'tracks'.
> music.album: Reverse query name for m2m field 'tracks' clashes with field
> 'Song.album'. Add a related_name argument to the definition for 'tracks'.
>
> uh oh. Now there's *TWO* errors!!! What am I doing wrong?
The point is that you don't need both fields. Django already provides a
reverse relationship whenever you define a foreign key or manytomany, and
it's this automatic attribute which is clashing. Remove the `album` FK from
the Song model (after all, if there's a manytomany from Song to Album, that
implies a song can be on mutliple albums, so there's no point in defining
the *one* album that a song is on, which is what the FK implies).
Now you can still get from Song to Album via `song.album_set.all().`
--
DR.
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