Changes between Initial Version and Version 1 of ManyToManyManager

Timestamp:

May 31, 2007, 3:31:35 PM (18 years ago)

Author:

Marty Alchin <gulopine@…>

Comment:

Initial document

ManyToManyManager

@@ +1 @@
+While the existing `ManyToManyField` is suitable for basic relationships, some projects find need to tie objects together along with some information about their relationship. The common example is the role an actor played in a movie, but it could be used for many other things, including what "base" a dating couple has gotten to, for instance. For these cases, the common recommendation is to simply create an intermediary model with `ForeignKey`s to each of the connected models, along with any extra fields that are appropriate for the relationship.
+
+In the Hollywood example, something like this:
+
+{{{
+#!python
+class Actor(models.Model):
+    name = models.CharField(maxlength=255)
+
+class Movie(models.Model):
+    title = models.CharField(maxlength=255)
+    actors = models.ManyToManyField(Actor)
+}}}
+
+would become something like this instead, with the `role` field added:
+
+{{{
+#!python
+class Actor(models.Model):
+    name = models.CharField(maxlength=255)
+
+class Film(models.Model):
+    title = models.CharField(maxlength=255)
+
+class Role(models.Model):
+    actor = models.ForeignKey(Actor, related_name='roles')
+    film = models.ForeignKey(Film, related_name='roles')
+    role = models.CharField(maxlength=255)
+}}}
+
+Unfortunately, the database API then goes from this:
+
+{{{
+#!python
+>>> for actor in film.actors.all():
+...     print actor.name
+Graham Chapman
+Terry Gilliam
+John Cleese
+Eric Idle
+}}}
+
+to this:
+
+{{{
+#!python
+>>> for role in film.roles.all():
+...     print '%s played %s' % (role.actor.name, role.role)
+Graham Chapman played King Arthur
+Terry Gilliam played Sir Bors
+John Cleese played Sir Lancelot the Brave
+Eric Idle played Sir Robin the Not-Quite-So-Brave-as-Sir Launcelot
+}}}
+
+While programmers may not have a problem with this change, it is very counter-intuitive for template authors who may not have (nor should they need) an intimate understanding of how relational databases work. I'd like to propose an alternative, using a custom manager for these intermediary models, that will enable a more natural API.
+
+By simply adding one line and renaming the `related_name`s:
+
+{{{
+#!python
+class Actor(models.Model):
+    name = models.CharField(maxlength=255)
+
+class Film(models.Model):
+    title = models.CharField(maxlength=255)
+
+class Role(models.Model):
+    actor = models.ForeignKey(Actor, related_name='films')
+    film = models.ForeignKey(Film, related_name='actors')
+    role = models.CharField(maxlength=255)
+
+    objects = models.ManyToManyManager()
+}}}
+
+we could get a much more intuitive API:
+
+{{{
+#!python
+>>> for actor in film.actors.all():
+...     print '%s played %s' % (actor.name, actor.role)
+Graham Chapman played King Arthur
+Terry Gilliam played Sir Bors
+John Cleese played Sir Lancelot the Brave
+Eric Idle played Sir Robin the Not-Quite-So-Brave-as-Sir Launcelot
+}}}
+
+This would work just as well in both directions:
+
+{{{
+#!python
+>>> for film in john.films.all():
+...     print '%s in %s' % (film.role, film.title)
+Sir Lancelot the Brave in Monty Python and the Holy Grail
+Sir Nicholas de Mimsy-Porpington in Harry Potter and the Sorceror's Stone
+}}}
+
+Ideally, this API would also extend the `add` method of the manager, allowing it to take keyword attributes for the relationship meta-data:
+
+{{{
+#!python
+>>> film = Film.objects.create(title='And Now for Something Completely Different')
+>>> john.films.add(film, role='Sir George Head')
+>>> for film in john.films.all():
+...     print '%s in %s' % (film.role, film.title)
+Sir Lancelot the Brave in Monty Python and the Holy Grail
+Sir Nicholas de Mimsy-Porpington in Harry Potter and the Sorceror's Stone
+Sir George Head in And Now for Something Completely Different
+}}}
+
+And without the intermediary model, there's need for an `update` function on the manager, which would handle modifying the meta-data:
+
+{{{
+#!python
+>>> film.actors.update(john, role='Mungo the Cook')
+}}}