Problem with NSPredicate in core data fetch request

Gideon King Sun, 13 Mar 2011 07:10:33 -0700

I'm seeing a strange thing with a predicate. When I apply the predicate to a 
fetch request, it doesn't return any results, but when I do the fetch without 
the predicate and then use filteredArrayUsingPredicate to filter the results, 
it has the matching row that I expect.


Here are the results of a query without any predicate:

(gdb) po [[topic managedObjectContext] executeFetchRequest:request error:&error]
<NSCFArray 0x16d15da0>(
<NMAttachmentMO: 0x19e3e780> (entity: Attachment; id: 0x1e095f40 
<x-coredata://75C22C98-BAF0-444B-BD95-A4F6B31153D4/Attachment/p39C7B8FF-EC6C-451B-9622-F3BDC8DCBB94>
 ; data: {
    displayName = "Import test map (picture)";
    fileName = "Import test map.png";
    nmObjectId = "72172A92-408D-41FA-AAB6-2132C4641AA5";
    resource = "0x1e06b400 
<x-coredata://75C22C98-BAF0-444B-BD95-A4F6B31153D4/Resource/pE9C234AD-2A42-4C87-973A-D6B61AD409DD>";
    topic = "0x1e0953b0 
<x-coredata://75C22C98-BAF0-444B-BD95-A4F6B31153D4/Topic/p5A7F2F62-3B71-434A-9C90-3C8116190D04>";
}),
<NMAttachmentMO: 0x150dace0> (entity: Attachment; id: 0x1e08ee10 
<x-coredata://75C22C98-BAF0-444B-BD95-A4F6B31153D4/Attachment/pC15B4925-7B19-45D9-9F0D-2CD0BF549DFD>
 ; data: {
    displayName = "example RSS feed opml";
    fileName = "rss_sharpreader.opml";
    nmObjectId = "DAC4868B-C8BA-41DB-9CC1-6F85193EC7B5";
    resource = "0x36f98c0 
<x-coredata://75C22C98-BAF0-444B-BD95-A4F6B31153D4/Resource/p2A467945-4674-472B-AA88-73FD47623EE4>";
    topic = "0x1e08dbd0 
<x-coredata://75C22C98-BAF0-444B-BD95-A4F6B31153D4/Topic/p63A290FB-EAA9-4750-955C-440352D50349>";
}),
<NMAttachmentMO: 0x1e165630> (entity: Attachment; id: 0x1e091ee0 
<x-coredata://75C22C98-BAF0-444B-BD95-A4F6B31153D4/Attachment/pCD474E6F-4AAA-4E6D-B2FC-478381F590AE>
 ; data: {
    displayName = nil;
    fileName = "import test mind map(novamind).png";
    nmObjectId = "784A6E81-2EEF-42F5-8D08-05F6B58A8B55";
    resource = "0x1e0fa240 
<x-coredata://75C22C98-BAF0-444B-BD95-A4F6B31153D4/Resource/p9D511092-DED8-4A29-94D6-8FC998108D82>";
    topic = "0x1e08d830 
<x-coredata://75C22C98-BAF0-444B-BD95-A4F6B31153D4/Topic/p7A88A2D6-9A96-4B24-BCF2-56FC62BB3D64>";
})
)


And here's my predicate: 

(gdb) po predicate
nmObjectId == "784A6E81-2EEF-42F5-8D08-05F6B58A8B55"



If I apply that predicate to the results of my fetch request:

(gdb) po [results filteredArrayUsingPredicate:predicate]
<NSCFArray 0xe71490>(
<NMAttachmentMO: 0x1e165630> (entity: Attachment; id: 0x1e091ee0 
<x-coredata://75C22C98-BAF0-444B-BD95-A4F6B31153D4/Attachment/pCD474E6F-4AAA-4E6D-B2FC-478381F590AE>
 ; data: {
    displayName = nil;
    fileName = "import test mind map(novamind).png";
    nmObjectId = "784A6E81-2EEF-42F5-8D08-05F6B58A8B55";
    resource = "0x1e0fa240 
<x-coredata://75C22C98-BAF0-444B-BD95-A4F6B31153D4/Resource/p9D511092-DED8-4A29-94D6-8FC998108D82>";
    topic = "0x1e08d830 
<x-coredata://75C22C98-BAF0-444B-BD95-A4F6B31153D4/Topic/p7A88A2D6-9A96-4B24-BCF2-56FC62BB3D64>";
})
)


So all that's as expected, but if I set exactly that predicate on my fetch 
request, no results match:

(gdb) call (void)[request setPredicate:predicate]
(gdb) po [[topic managedObjectContext] executeFetchRequest:request error:&error]
<NSCFArray 0x37b5a50>(
)


I am really mystified by this - I can't understand why it would not work when 
applied to a fetch request, but would work when applied to the array of results.

Any suggestions?


Here's my code which returns 0 rows:

        NSString *attachmentID = [[url path] lastPathComponent];
        
        NSFetchRequest *request = [[NSFetchRequest alloc] init];
        NSError *error = nil;
        NSEntityDescription *entity = [NSEntityDescription 
entityForName:kNMAttachmentEntityKey inManagedObjectContext:[topic 
managedObjectContext]];
        
        [request setEntity:entity];
        NSPredicate *predicate = [NSPredicate predicateWithFormat:@"nmObjectId 
== %@", attachmentID];
        [request setPredicate:predicate];

        NSArray *results = [[topic managedObjectContext] 
executeFetchRequest:request error:&error];
        [request release];






Regards

Gideon






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Problem with NSPredicate in core data fetch request

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