Why “NOT IN” does not work in this NSPredicate?

问题:

A.b and B.a are inverse to-many relationships. Why does this predicate for A work:

NSPredicate *predicate = [NSPredicate predicateWithFormat:@"NOT SELF IN %@", bObject.a]; 

while this one does not:

NSPredicate *predicate = [NSPredicate predicateWithFormat:@"NOT %@ IN b", bObject]; 

I think both predicates should give the same result ― the collection of As that have no relation with bObject via a<-->b. But in fact, the first one gives the correct collection while the second one not.

Update:

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Here is a sample project wherein A.b is embodied by Account.filtered_clients and B.a is embodied by Client.filtered_by.

Toggle commenting of line 143 and line 144 in MasterViewController.m to see the difference.

Please help me either find the bug in my code, or confirm it is a Core Data bug so I can report it to Apple. Thanks very much.

回答1:

Here is another idea: maybe it is trying to "negate" the bObject with the NOT. Thus, try:

@"NOT (%@ IN b)" 

回答2:

From the documentation:

Important You must define many-to-many relationships in both directions―that is, you must specify two relationships, each being the inverse of the other. You can’t just define a to-many relationship in one direction and try to use it as a many-to-many. If you do, you will end up with referential integrity problems.

So check this:

1. A has a to many relationship to B.
2. B has a to many relationship to A.
3. The A relationship is the inverse of the B relationship.
4. The B relationship is the inverse of the A relationship.

回答3:

I think I found the answer for people coming across this question. Apparently you do need to use SELF for it to work, at least in my case with two to-many relations. This works for me:

NOT (SELF IN %@) 

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