#30543: admin.E108 is raised on fields accessible only via instance.
               Reporter:  ajcsimons             |          Owner:  nobody
                   Type:  Bug                   |         Status:  new
              Component:  Core (System checks)  |        Version:  master
               Severity:  Normal                |       Keywords:
           Triage Stage:  Accepted              |      Has patch:  0
    Needs documentation:  0                     |    Needs tests:  0
Patch needs improvement:  0                     |  Easy pickings:  0
                  UI/UX:  0                     |
 #28490 caused a regression in 47016adbf54b54143d4cf052eeb29fc72d27e6b1,

 1. if hasattr(obj.model, item) returns false then we go straight to the
 else clause which returns the error,
 2. whereas before the else clause did another check for
 model._meta.get_field(item) and would only return the error if that raised
 a FieldDoesNotExist exception
 3. So how is it that hasattr(model, item) can return false, but
 model._meta.get_field(item) will return something meaning the check should
 not return an error?
 4. Answer: the field is a special one which is only valid to access on
 instances of the model (whose ModelAdmin we are verifying) and not the
 model class itself. An example of this is the PositionField from the
 django-positions library (which inherits from djangos
     def __get__(self, instance, owner):
         if instance is None:
             raise AttributeError("%s must be accessed via instance." %
         current, updated = getattr(instance, self.get_cache_name())
         return current if updated is None else updated
 5. So the simplification of the hasattr branch was valid, but the removal
 of the else branch to no longer check get_field doesn't throw before
 returning an error was not a refactor but a functionality altering change
 which makes this method return errors in cases where it used not to.

Ticket URL: <https://code.djangoproject.com/ticket/30543>
Django <https://code.djangoproject.com/>
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