Core Data: NSPredicate for many-to-many relationship. ("to-many key not allowed here")

Question

I have two entities named "Category" and "Article" which have a many to many relationship. I want to form a predicate which searches for all articles where category.name is equal to some value. I have the following:

 NSEntityDescription  *entityArticle   = [NSEntityDescription entityForName:@"Article" inManagedObjectContext:managedObjectContext]; 
 NSSortDescriptor  *sortDescriptor   = [[NSSortDescriptor alloc] initWithKey:@"title" ascending:YES];
 NSArray     *sortDescriptors  = [[NSArray alloc] initWithObjects:sortDescriptor, nil];
 NSPredicate    *predicate    = [NSPredicate predicateWithFormat:@"categories.name == [cd] %@", category.name]; 

 [request setSortDescriptors:sortDescriptors];
 [request setEntity:entityArticle];
 [request setPredicate:predicate];

 NSMutableArray *results = [[managedObjectContext executeFetchRequest:request error:nil] mutableCopy];

 if ([results count] > 0)
  NSLog(@"Results found."); 
 else 
  NSLog(@"NO results found."); 

 [request release];
 [sortDescriptor release];
 [sortDescriptors release];

The error I receive is *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'to-many key not allowed here'

Are there any options to retrieve the desired data?

A SUBQUERY may also be what you need. See: http://stackoverflow.com/questions/11346546/core-data-unsupported-predicate-with-all-and-in — Collin, Aug 16 '13 at 12:44

score 162 · Accepted Answer · answered Nov 18 '10 at 17:47

162

You're trying to compare a collection (categories.name) to a scalar value (category.name). You need to either use a collection comparator (CONTAINS), or use a predicate modifier (ANY/ALL/SOME, etc).

Try using:

[NSPredicate predicateWithFormat:@"ANY categories.name =[cd] %@", category.name];

Or:

[NSPredicate predicateWithFormat:@"categories.name CONTAINS[cd] %@", category.name];

answered Nov 18 '10 at 17:47

Dave DeLong

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This works for me, both with and without the '[cd]'. Can you elaborate on the function of that? – Duncan Babbage Nov 11 '11 at 08:07
17

@DuncanBabbage the `[cd]` means to do a case-insensitive (the `c`) and a diacritic-insensitive (the `d`) search. – Dave DeLong Nov 11 '11 at 18:29
3

It's not working if `category` has inverse `to one` relation to 'Article'. Any idea how to predicate that ? – iEngineer May 13 '14 at 10:32

score 9 · Answer 2 · answered Mar 20 '18 at 04:54

9

SWIFT SYNTAX

In case anyone happens upon this writing in swift as I did...

let predicate = NSPredicate(format: "ANY categories.name = %@", category.name!)
fetchRequest.predicate = predicate

worked for me.

answered Mar 20 '18 at 04:54

caleb81389

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1
7

score 0 · Answer 3 · answered Feb 17 '20 at 11:18

0

For those who faced the same problem. You can use IN operator instead of = / == to look for specific objects in any Collection. Look here for reference. No difference Swift or Objective-C

answered Feb 17 '20 at 11:18

Oleh H

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10
18

Core Data: NSPredicate for many-to-many relationship. ("to-many key not allowed here")

3 Answers3

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