I was surprised to see a Core Data object throw an exception during
'processPendingChanges' because the object couldn't handle
'setValue:nil forKey:'.
AFAICT, this happened under the following conditions:
-- the object had been inserted into the managed object context
-- the object had then been deleted from the context (some of the
objects being created eventually get deleted)
-- no save had ever been done to the persistent store
-- 'processPendingChanges' was in the middle of faulting out the
object that had been deleted
-- 'processPendingChanges' was trying to set every attribute of the
object to nil
-- the attribute in question was of type 'undefined' and marked
transient
-- there were custom accessors for the attribute's key that get and
set it as a NSRect
(I don't think it matters how the object came to be deleted -- I
suspect that the same exception would occur whenever it was deleted
from the context. Also, the undefined/transient attribute with custom
scalar/struct accessors is a standard Core Data coding pattern. I
copied the code from the documentation.)
What's surprising about this is Core Data's apparent assumption that
an 'undefined' attribute is nevertheless represented by some kind of
Objective C object, and is therefore setting it to nil is safe.
Clearly, in this case, there is no object and setting the attribute to
nil is senseless, and I would have expected Core Data not to try
anything tricky like this precisely because the attribute's type is
'undefined'.
So is this to be regarded as a bug in Core Data? A bug in my
understanding?
(I had earlier noticed and worked around a cousin to this issue: if a
transient/undefined attribute is implemented by a managed object that
can return nil for the corresponding key, then there's a validation
error at save time if the attribute is not also marked optional.
Again, if the attribute type is 'undefined', I would have expected
Core Data to make no assumptions about how to interpret the value of
the key.)
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