NSPredicate filter to-Many with child object property

问题

I have what I thought was a simple problem. I am attempting to filter some core data where I have a Parent object which has a to-many relationship with a child object and that child object has a string id. I want to get all the parent objects where no child object has a specific id.

I have tried !(ANY... LIKE) as well as !(ANY..==) and NONE with like and == and ALL children.id != otherid

My querying looks like:

NSFetchRequest* fetchRequest = [NSFetchRequest fetchRequestWithEntityName:@"Parent"];

NSPredicate* predicate = [NSPredicate predicateWithFormat:@"NONE children.id LIKE %@",otherID];
[fetchRequest setPredicate: predicate];
NSError* error;
NSArray* allParents = [[DataManager context] executeFetchRequest:fetchRequest error:&error];
//sanity check the predicate
for (Parent* p in allParents) {
    for (Child* c in p.children) {
        if([c.id isEqualToString:otherID]){
            NSLog(@"PREDICATE FAIL!");
        }
    }
}

Am I missing something with NSPredicate? Is this type of filtering allowed for CoreData? Better solution?

回答1:

I found a similar question although not easily apparent. It turns out the answer is the tricky SUBQUERY. Here is what led me on the chase:

NSPredicate Aggregate Operations with NONE

and an more open explanation about SUBQUERY here:

http://funwithobjc.tumblr.com/post/2726166818/what-the-heck-is-subquery

The resulting predicate is:

//in children get a $child and group all the $child objects together
//where the ids match, if that groups count is 0 we know 
//the parent has no child with that id
[NSPredicate predicateWithFormat:
   @"SUBQUERY(children, $child, $child.id == %@).@count == 0",objectId];

来源：https://stackoverflow.com/questions/20235381/nspredicate-filter-to-many-with-child-object-property

标签

ios

objective-c

core-data

nspredicate