You don't need the temporary field. 1) create the "through" model, add it to the migration file 2) migrate all m2m instances (e.g. iterate over all Bereich objects then iterate through its users, creating a UserBereichAssignment object for each (all this done in a migrations.RunPython call) 3) change the m2m field by adding the through option, and add this change to the migrations file.
However, you may want to go with django-guardian and thus, with object-level permissions instead ;) On 16 Jul 2015 16:05, "Carsten Fuchs" <carsten.fu...@cafu.de> wrote: > Dear Django fellows, > > using Django 1.8.3, in a fully migrated app I have a model with a > many-to-many relationship like this: > > from django.db import models > from django.contrib.auth.models import User > > class Bereich(models.Model): > benutzer = models.ManyToManyField(User) > > Now I would like to store some extra information with the relationship > (e.g. some Booleans that describe what the user is permitted to do with the > related Bereich object). > That is, the intended result is planned to look like this: > > class Bereich(models.Model): > benutzer = models.ManyToManyField(User, > through='UserBereichAssignment') > > class UserBereichAssignment(models.Model): > bereich = models.ForeignKey(Bereich) > user = models.ForeignKey(User) > can_edit = models.BooleanField(verbose_name="can edit?", > default=True) > > However: > > $ ./manage.py makemigration # ok, creates a new migrations file > $ ./manage.py migrate > > # ... > ValueError: Cannot alter field Lori.Bereich.benutzer into > Lori.Bereich.benutzer - they are not compatible types (you cannot alter to > or from M2M fields, or add or remove through= on M2M fields) > > > Well, I understand that and also seem to see some of the involved > problems: For example, how would it make sure that the same database table > is used for the previous implicit intermediate model and the new > UserBereichAssignment? > > I'm also aware of https://code.djangoproject.com/ticket/23034, but I'm > not sure what to make of it, or if it is even applicable here. > > > Thus, what would be a good way to proceed? > > To the best of my understanding, I could do this manually, that is: create > an entirely new many-to-many field, e.g. > > class Bereich(models.Model): > benutzer = models.ManyToManyField(User) # old, > unchanged > benutzer_ = models.ManyToManyField(User, > through='UserBereichAssignment') > > then manually migrate the data from "benutzer" to "benutzer_" (possibly > using a data migration?), and finally remove the old field "benutzer". The > only downside seems that all dependent code had to be updated from using > the old name "benutzer" to the new name "benutzer_". (I'm not sure if > making a migration for renaming the new field name to the old field name is > possible?) > > Alas... are there any viable alternatives to this? > I'd be very grateful for any hint! :-) > > Best regards, > Carsten > > -- > You received this message because you are subscribed to the Google Groups > "Django users" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to django-users+unsubscr...@googlegroups.com. > To post to this group, send email to django-users@googlegroups.com. > Visit this group at http://groups.google.com/group/django-users. > To view this discussion on the web visit > https://groups.google.com/d/msgid/django-users/55A7BA3E.5030105%40cafu.de. > For more options, visit https://groups.google.com/d/optout. > -- You received this message because you are subscribed to the Google Groups "Django users" group. To unsubscribe from this group and stop receiving emails from it, send an email to django-users+unsubscr...@googlegroups.com. To post to this group, send email to django-users@googlegroups.com. Visit this group at http://groups.google.com/group/django-users. To view this discussion on the web visit https://groups.google.com/d/msgid/django-users/CACczBUKez8kCfMFJgT94P_3bueNrdYck7fCqWQWF6hO_1AtEFw%40mail.gmail.com. For more options, visit https://groups.google.com/d/optout.
