核心数据，NSPredicate和to-many密钥

Question

I have a Core Data model in which a Task entity includes an optional to-many relationship excludedOccurrences. One of the properties of excludedOccurrences is start, which is an NSDate object. The ExcludedOccurrence entity has an inverse mandatory to-one relationship to the Task entity.

In order to fetch tasks for a specified day, I need to make sure that the specified day does not appear as the start property of any ExcludedOccurrence entity. One of the sub-predicates I am trying to use is therefore

NSPredicate *occurrenceIsNotExcludedPredicate = [NSPredicate predicateWithFormat: @"(ALL excludedOccurrences.start != %@))", today];

where today is a NSDate object for today including only the day, month and year components. All of the excluded occurrences start properties also include just the day, month and year components.

While this should fine, at least reading the documentation for Core Data and NSPredicate, I get the following error message:

Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'Unsupported predicate

If I use the equivalent predicate

NSPredicate *occurrenceIsNotExcludedPredicate = [NSPredicate predicateWithFormat: @"!(ANY excludedOccurrences.start == %@))", today];

no exception is thrown, however the code does not work as expected: the occurrence for today, which should not be excluded, is instead excluded.

I am not sure also how to test for the case excludedOccurrences == nil: the following predicate

NSPredicate *nilPredicate = [NSPredicate predicateWithFormat: @"(excludedOccurrences == nil)"];

causes at runtime the exception

to-many key not allowed here

However, since the excludedOccurrences relationship is optional, I also need to test if it is nil.

How do I deal with this? Thank you in advance.

Answer 1

To test for an empty relationship you should compare the count of the to-many key to zero.

[NSPredicate predicateWithFormat:@"excludedOccurrences.@count == 0"];

As for your subpredicates, be aware that you can only have one of either the ALL or ANY modifiers in your final predicate, although you can use that modifier multiple times throughout the predicate.

Not OK : ANY foo.bar = 1 AND ALL foo.baz = 2
OK: ANY foo.bar = 1 AND !(ANY foo.baz != 2)

Answer 2

with the help of you all, I finally managed to determine the correct predicate for my scenario. It looks like that an NSDate object is handled as a double, however, the double is never something like 3.7, it is always like 3.0 Therefore, the following predicate correctly works in my tests:

NSPredicate *occurrenceIsNotExcludedPredicate = [NSPredicate predicateWithFormat: @"(excludedOccurrences.@count == 0 || (excludedOccurrences.@count > 0 && NONE excludedOccurrences.start == %@))",thisDate];

where thisDate is a NSDate object containing only the day, month and year components (as in the case of the start property of the ExcludedOccurrence entity.

Testing for an empty relationship is basically done using the @count aggregate operator, as suggested by some folks at Apple.

Again, thankyou very much for your help. I still observe that the documentation is flawed in several parts (especially where it says that ALL works fine while, instead, it does not work at all).

Answer 3

So, to test for a non-empty relationship, this actually works:

[NSPredicate predicateWithFormat:@"relationship.@count != 0"]

The solution given by Ashley Clark crashes for me giving "to-many key not allowed here"

Answer 4

在swift 2中，类似于：

request.predicate = NSPredicate(format: " relationship.@count != 0")