Documentation example of customising model instance loading has a bug

[Ali Toosi]

The docs provide an example of how the model instance loading can be changed and how _loaded_values can be saved for future comparison here: ​https://docs.djangoproject.com/en/4.0/ref/models/instances/

In this example, it checks if there are any values not loaded from db then add them as DEFERRED values so class instantiation would work (cls(*values)). However, this would mean values list has items now that do not map to any field_names so when at the end of the function, we zip them together and store in _loaded_values, the code will fail.

The easiest fix for this is to update the field_names too so they would 1. work and 2. show which fields were not loaded from the db at the time.

I've opened a PR for this here: ​https://github.com/django/django/pull/15664

[Mariusz Felisiak]

Thanks for the ticket.

The easiest fix for this is to update the field_names too so they would 1. work and 2. show which fields were not loaded from the db at the time.

I'd keep the previous behavior and rather filter out DEFERRED from the list of values, e.g.

docs/ref/models/instances.txt

@@ -103 +103 @@
         instance._state.adding = False
         instance._state.db = db
         # customization to store the original field values on the instance
-        instance._loaded_values = dict(zip(field_names, values))
+        instance._loaded_values = dict(
+            zip(field_names, (value for value in values if value is not DEFERRED))
+        )
         return instance
 
     def save(self, *args, **kwargs):

[Ali Toosi]

Replying to Mariusz Felisiak:
Thanks, Mariusz. I've updated the PR with your suggestion.

[Mariusz Felisiak <felisiak.mariusz@…>]

In faab9e67:

Fixed #33680 -- Corrected example of customizing model loading in docs.

[Mariusz Felisiak <felisiak.mariusz@…>]

In 8b2a93ee:

[4.0.x] Fixed #33680 -- Corrected example of customizing model loading in docs.

Backport of faab9e6769b01c18d9e3a31504601452eede6150 from main