I received reports of identical Core Data validation errors from two users in
the last week, which I cannot reproduce. It involves these relationships,
which are inverses of one another:
Bar Foo
foo <<----> bars
In both directions, the Delete Rule is "Nullify", and for the to-many relation,
Min Count and Max Count are "none". Bar's 'foo' is optional. Foo's bars are
not optional, although that does not seems to be the problem since all of the
errant Foo objects in the reports seem to have bars.
The reports indicate (rather strangely, but I'm watching my word count here)
that the problem is "Dangling reference to an invalid object". Searching list
archives tells me that this has been due to setting a relationship in
-awakeFromFetch, which is illegal. But I checked my project carefully and I'm
not doing that.
However, in the error reports, I see that only one of the foo:bar relationships
of the Foo objects tagged as NSValidationErrorObject are fulfilled: The Foo's
'bar' contains several Bar objects, but these Bar objects' 'foo' is nil. I
think that's the cause of the validation error – Core Data expects pairs of
inverse relationships with Delete Rules both set to Nullify to always be either
point to one another or both be nil.
Indeed, I cannot reproduce this 1/2 relationship in debugging…
• First, I try to do it by only creating one half. When my app creates a Foo,
it adds a couple of Bar objects immediately. When I do that in debugging, Core
Data immediately fulfills the inverse, so that the Bar objects now have a foo.
So I have a two relationships.
• Then, I try to do it by deleting one half. I grab a Bar which has a 'foo',
and set send it setFoo:nil. Core Data immediately removes this Bar object from
the 'bars' set of its former Foo object. So I have no relationships.
Summarizing, the paradox is that there I cannot, in code, create a 1/2
relationship, apparently because of the data model settings. But it happens
occasionally in the wild. How?
Thanks,
Jerry Krinock
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