Core data exception "Couldn't apply expression side effects : Couldn't dematerialize a result variable: couldn't read its memory"

Question

Has anyone seen the source of this Core data exception? What coulds be the cause? Thanks

Core data will throw an exception but continue without any error

Printing the exception breakpoint:

(lldb) po *(id *)($esp + 4)
error: Couldn't apply expression side effects : Couldn't dematerialize a result variable: couldn't read its memory

Exception is thrown when calling save from a child managed object context

__block NSError *error;
[backgroundContext performBlockAndWait:^{
    [backgroundContext save:&error];
}];

Answer 1

This is due to a constraint violation. If you do a po [$arg1 reason] you should get constraint validation failure meaning that the uniqueness constraint on one of your objects has been violated. How this is then handled will depend on your merge policy.