Support get_FOO_display() on GeneratedField

[Ronie Martinez]

Please add support for get_FOO_display() on GeneratedField.

We initially used a normal IntegerField with choices [IntegerField(choices=choices, ...)] and then running a SQL to make this field a generated field. We then migrated to GeneratedField while maintaining the output_field similar to our old one.

However, we found that get_FOO_display() don't work anymore and we had to write a method for this, instead.

[Sarah Boyce]

Hello Ronie!

I think that the use of "choices" in a GeneratedField is key here as there's some specific logic for get_FIELD_display() when choices is set.

For me to be sure, can you share what error you received here (or some more specifics on how it didn't work if there's no error)?

If you got an error like: AttributeError: 'Bar' object has no attribute 'get_FOO_display'. Did you mean: 'get_something_else_display'?

Then I think I understand the issue and have replicated (sample failing test attached below).

It's also possible you received this error: AttributeError: Cannot read a generated field from an unsaved model. if get_FOO_display was called before the model saved (related to #35560). If this is the issue, can you share a bit more about when this gets called? Maybe sharing your model's GeneratedField would also help 🙂

tests/model_fields/models.py

@@ -72 +72 @@
     c = models.CharField(choices=iter(()), blank=True, max_length=1)
 
 
+class WhizGenerated(models.Model):
+    c = models.IntegerField(choices=Whiz.CHOICES, null=True)
+    c_copy = models.GeneratedField(
+        expression=F("c"),
+        output_field=models.IntegerField(choices=Whiz.CHOICES, null=True),
+        db_persist=True,
+    )
+
+    class Meta:
+        required_db_features = {"supports_stored_generated_columns"}
+
+
 class Choiceful(models.Model):
     class Suit(models.IntegerChoices):
         DIAMOND = 1, "Diamond"

tests/model_fields/tests.py

@@ -17 +17 @@
     WhizDelayed,
     WhizIter,
     WhizIterEmpty,
+    WhizGenerated,
 )
 
 
@@ -297 +298 @@
         self.assertEqual(WhizIterEmpty(c="").c, "")  # Empty value
 
 
+class GetFieldDisplayGeneratedTests(TestCase):
+    def test_choices_and_field_display(self):
+        wg_0 = WhizGenerated.objects.create(c=0)
+        wg_1 = WhizGenerated.objects.create(c=1)
+        wg_none = WhizGenerated.objects.create(c=None)
+        self.assertEqual(wg_0.get_c_copy_display(), "Other")
+        self.assertEqual(wg_1.get_c_copy_display(), "First")
+        self.assertIsNone(wg_none.get_c_display())
+
+
 class GetChoicesTests(SimpleTestCase):
     def test_empty_choices(self):
         choices = []

[Sarah Boyce]

Read through the docs for ​Model.get_FOO_display()
As this technically doesn't imply this will be supported for a GeneratedField, agree this can be considered a new feature and not a release blocker 👍

[Ronie Martinez]

Hi Sarah,

We are getting 'MyModel' object has no attribute 'get_FOO_display'.

Thanks for accepting this feature.

[Sarah Boyce]

Replying to Ronie Martinez:

Thanks for accepting this feature.

You're very welcome, thank you for raising 👍