Changes between Version 18 and Version 19 of ContribAuthImprovements

Timestamp:

Apr 4, 2012, 1:08:29 AM (14 years ago)

Author:

Russell Keith-Magee

Comment:

Added Option 2c; generic swappable models.

ContribAuthImprovements

@@ -173 +173 @@
  * !ModelForm (and any other code that introspects auth.User) should be made lazy, or else circular dependencies can result. Introspecting auth.User and plugging into auth.User from the same app or an app loaded later is a potential circular dependency. (This is not a problem, unless you put mixins in models.py?)
 
+== Solution 2c: Generic swappable models ==
+
+Follows the general direction of Solution 2a, but instead of a `USER_MODEL` setting that only solves the problem for auth.User, sets up the infrastructure for the general concept of swappable models.
+
+== Implementation ==
+
+A model that wants to declare itself as swappable adds a new Meta option:
+
+{{{
+class User(Model):
+    ....
+    class Meta:
+        swappable = 'user'
+}}}
+
+Here, the name 'user' is an identifier that will be used to refer to this swappable model. By convention, it might be a good idea to namespace this tag (e.g., 'auth.user', instead of just 'user'. 
+
+We then introduce a `SWAPPABLE_MODELS` setting that provides a way for users to specify which models will be overridden in this application:
+
+{{{
+SWAPPABLE_MODELS = {
+    'user': 'myapp.SuperDuperUser'
+}
+}}}
+This specifies that the 'user' model will be satisfied by !SuperDuperUser in this project.
+
+If a model identifier (i.e., 'user') is mentioned in `SWAPPABLE_MODELS`, then the original model (auth.User) isn't synchronised to the database, and isn't added to the App cache.
+
+We then add a !LazyForeignKey():
+{{{
+class Comment(Model):
+    user = LazyForeignKey('user')
+    ....
+}}}
+that will resolve the !ForeignKey to the currently defined swappable model.
+
+If an app defines a model with a !ForeignKey to a swappable model, a warning is raised (since the model could potentially be swapped out); if the model is currently defined as swapped out, then an error will be raised (since the ForeignKey will be pointing at the wrong table).
+
+The app cache gains a registry of swappable models, and a `get_swappable_model(identifier)` entry point, so that users can easily retrieve the model currently being used as the swappable model for the various swappable endpoints.
+
+=== Advantages ===
+
+As for Solution 2a, but:
+
+ * Solves the general problem of swappable models, so other apps with similar problems (e.g., comments) can use the same infrastructure instead of having to reinvent the wheel.
+ * Catches migration problems as warnings/errors if existing apps haven't been updated to point to the swappable model.
+
+=== Problems ===
+
+As for Solution 2a.
+
 == Solution 3: Leverage App Refactor ==