Re: Trying to figure out ManyToManyField on a legacy database

Hi Brian,

It seems like you don't really have an Many To Many relationship between your "Song" and "Played" objects.  I can see how you would have multiple plays for one song, but considering that the Played model has a track_id, would you ever have multiple songs for a single Play?    It appears that "track_id" is intended to be a foreign key into the Song table, so maybe you could consider using a ForeignKey field to refer to your Song through your Played object.  Django will automatically create 2-way references so you can get to your played objects from your song object::

class Played(models.Model):
  played_id = models.IntegerField(primary_key=True)
  track = models.ForeignKey('Song')
  date_played = models.DateTimeField()
  played_by = models.CharField(max_length=255L)
  played_by_me = models.IntegerField()
  class Meta:
   db_table = 'played'

class Song(models.Model):

  id = models.IntegerField(primary_key=True)
  title = models.CharField(max_length=255L, blank=True)

      class Meta:

          db_table = 'song'

Then, you should be able to get all of the Plays for a particular song::  

s = Song.objects.get(id=1229)

s.title # should be OK still

s.played_set.all() # should be a list of plays.

On Saturday, May 11, 2013 5:33:46 PM UTC-5, Brian Millham wrote:

Hi all,
I'm just trying to learn Django. I have an existing site that I wrote in PHP, and I'l considering converting it to Django,

I have a legacy database that I don't want to make any changes to. I have the basics working in Django, but am having a problem with a ManyToManyField.

Here are the 2 tables (simplified)

class Played(models.Model):
  played_id = models.IntegerField(primary_key=True)
  track_id = models.IntegerField()
  date_played = models.DateTimeField()
  played_by = models.CharField(max_length=255L)
  played_by_me = models.IntegerField()
  class Meta:
   db_table = 'played'

class Song(models.Model):
  id = models.IntegerField(primary_key=True)
  title = models.CharField(max_length=255L, blank=True)
  #These are what I've tried for ManyToMany with the error
  played = models.ManyToManyField(Played, db_column='id') # DatabaseError: (1146, "Table 'ampache.song_played' doesn't exist")
  played = models.ManyToManyField(Played, db_column='track_id') # DatabaseError: (1146, "Table 'ampache.song_played' doesn't exist")
  played = models.ManyToManyField(Played, db_table = 'played', db_column='id') # DatabaseError: (1054, "Unknown column 'T2.song_id' in 'where clause'")
  played = models.ManyToManyField(Played, db_table = 'played', db_column='track_id') # DatabaseError: (1054, "Unknown column 'T2.song_id' in 'where      clause'")
  played = models.ManyToManyField(Played, through = 'played', db_column='id') # AttributeError: 'ManyToManyField' object has no attribute '_m2m_reverse_name_cache'
  class Meta:
   db_table = 'song'

The code to produce the errors:

from djmusic.models import Song, Played
s = Song.objects.get(id=1229)
print s.title # Prints the title correctly
print s.played.all() # Generates one of the above errors

I know it's gotta be something simple I'm missing here. If I can get this figured out, there's a good chance that I'll convert my old PHP based site to Django.

Thanks all!
 

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