Core Data fetched property $FETCHED_SOURCE resolves to an object ID

Question

In a Core Data fetched property expression, apparently $FETCH_SOURCE resolves to an object ID instead of the object itself. This caused an exception "class is not key-value coding...". Which is true since it's the object that responds to that key.

I would like to use the source object's property value(s) as part of a fetched property's predicate.

Any idea how to fix this?

*** Terminating app due to uncaught exception 'NSUnknownKeyException', reason: '[<_NSObjectID_48_0 0x10101b200> valueForUndefinedKey:]: this class is not key value coding-compliant for the key messageType.'
*** First throw call stack:
(
    0   CoreFoundation                      0x00007fff92ba00c6 __exceptionPreprocess + 198
    1   libobjc.A.dylib                     0x00007fff8f85e3f0 objc_exception_throw + 43
    2   CoreFoundation                      0x00007fff92c34249 -[NSException raise] + 9
    3   Foundation                          0x00007fff9518b08c -[NSObject(NSKeyValueCoding) valueForUndefinedKey:] + 238
    4   Foundation                          0x00007fff950d2df9 -[NSObject(NSKeyValueCoding) valueForKey:] + 400
    5   Foundation                          0x00007fff950f36a8 -[NSObject(NSKeyValueCoding) valueForKeyPath:] + 341
    6   Foundation                          0x00007fff950a3489 -[NSFunctionExpression expressionValueWithObject:context:] + 751
    7   CoreData                            0x00007fff88cbbdda -[NSSQLSimpleWhereIntermediate initWithPredicate:inScope:] + 618
    8   CoreData                            0x00007fff88cbb672 -[NSSQLGenerator generateWhereIntermediatesInContext:] + 162
    9   CoreData                            0x00007fff88cbb010 -[NSSQLGenerator generateIntermediatesForFetchInContext:countOnly:] + 416
    10  CoreData                            0x00007fff88cb7d30 -[NSSQLGenerator newSQLStatementForFetchRequest:ignoreInheritance:countOnly:nestingLevel:] + 400
    11  CoreData                            0x00007fff88cb7a97 -[NSSQLAdapter _newSelectStatementWithFetchRequest:ignoreInheritance:] + 471
    12  CoreData                            0x00007fff88cb76b6 -[NSSQLCore newRowsForFetchPlan:] + 118
    13  CoreData                            0x00007fff88cb6f2e -[NSSQLCore objectsForFetchRequest:inContext:] + 526
    14  CoreData                            0x00007fff88cb6a91 -[NSSQLCore executeRequest:withContext:error:] + 225
    15  CoreData                            0x00007fff88cb5f72 -[NSPersistentStoreCoordinator executeRequest:withContext:error:] + 2114
    16  CoreData                            0x00007fff88cb4309 -[NSManagedObjectContext executeFetchRequest:error:] + 537
    17  CoreData                            0x00007fff88cfca4e -[NSManagedObjectContext(_NestedContextSupport) _parentObjectsForFetchRequest:inContext:error:] + 414
    18  CoreData                            0x00007fff88d3072a __82-[NSManagedObjectContext(_NestedContextSupport) executeRequest:withContext:error:]_block_invoke_0 + 634
    19  libdispatch.dylib                   0x00007fff8b2710b6 _dispatch_client_callout + 8
    20  libdispatch.dylib                   0x00007fff8b272723 _dispatch_barrier_sync_f_invoke + 39
    21  CoreData                            0x00007fff88cfc87c _perform + 172
    22  CoreData                            0x00007fff88cfc6c2 -[NSManagedObjectContext(_NestedContextSupport) executeRequest:withContext:error:] + 354
    23  CoreData                            0x00007fff88cb4309 -[NSManagedObjectContext executeFetchRequest:error:] + 537
    24  CoreData                            0x00007fff88d0481e -[NSFaultHandler retainedFulfillAggregateFaultForObject:andRelationship:withContext:] + 254
    25  CoreData                            0x00007fff88d7faa8 -[_NSFaultingMutableArray willRead] + 120
    26  CoreData                            0x00007fff88d7fe99 -[_NSFaultingMutableArray count] + 25
    27  CoreFoundation                      0x00007fff92b82e69 -[NSArray lastObject] + 25
    28  Scuttlebutt                         0x0000000100041aab -[BSMessageCellView setObjectValue:] + 283
    29  AppKit                              0x00007fff936ca006 -[NSTableRowData _addViewToRowView:atColumn:row:] + 696
    30  AppKit                              0x00007fff936c9b5f -[NSTableRowData _addViewsToRowView:atRow:] + 151
    31  AppKit                              0x00007fff936c80f5 -[NSTableRowData _addRowViewForVisibleRow:withPriorView:] + 415
    32  AppKit                              0x00007fff936c7e7a -[NSTableRowData _addRowViewForVisibleRow:withPriorRowIndex:inDictionary:withRowAnimation:] + 272
    33  AppKit                              0x00007fff936c7149 -[NSTableRowData _unsafeUpdateVisibleRowEntries] + 740
    34  AppKit                              0x00007fff936c6ce1 -[NSTableRowData updateVisibleRowViews] + 119
    35  AppKit                              0x00007fff936da8b3 -[NSTableRowData _idleUpdateVisibleRows] + 66
    36  CoreFoundation                      0x00007fff92b5cdc4 __CFRUNLOOP_IS_CALLING_OUT_TO_A_TIMER_CALLBACK_FUNCTION__ + 20
    37  CoreFoundation                      0x00007fff92b5c8dd __CFRunLoopDoTimer + 557
    38  CoreFoundation                      0x00007fff92b420c9 __CFRunLoopRun + 1513
    39  CoreFoundation                      0x00007fff92b416e2 CFRunLoopRunSpecific + 290
    40  HIToolbox                           0x00007fff9427e0a4 RunCurrentEventLoopInMode + 209
    41  HIToolbox                           0x00007fff9427de42 ReceiveNextEventCommon + 356
    42  HIToolbox                           0x00007fff9427dcd3 BlockUntilNextEventMatchingListInMode + 62
    43  AppKit                              0x00007fff93544b13 _DPSNextEvent + 685
    44  AppKit                              0x00007fff935443d2 -[NSApplication nextEventMatchingMask:untilDate:inMode:dequeue:] + 128
    45  AppKit                              0x00007fff9353b773 -[NSApplication run] + 517
    46  AppKit                              0x00007fff934e01a6 NSApplicationMain + 869
    47  Scuttlebutt                         0x0000000100002225 main + 53
    48  Scuttlebutt                         0x00000001000021e4 start + 52
    49  ???                                 0x0000000000000003 0x0 + 3
)

This is what came up when I tried dumping the offending object's details -- quite visible that it's an object ID instead of the object itself.

(lldb) expression (NSString*) [(id)0x10101b200 description]
(NSString *) $0 = 0x0000000108b03880 @"0x10101b200 <x-coredata://8F1FBB6B-505B-4169-A9D0-10D48CE5D4DC/YammerMessage/p101>"
(lldb) expression (Class) [(id)0x10101b200 class]
(Class) $2 = _NSObjectID_48_0

Answer 1

You can set the result type of a NSFetchRequest with the method setResultType:, to get NSManagedObjects, you have to set it to NSManagedObjectResultType.

NSFetchRequest *fetchrequest = [[NSFetchRequest alloc] init];
[fetchRequest setResultType:NSManagedObjectResultType];

If you use the CoreData editor of XCode, you can choose the result type by selecting the fetch request and navigating to the 'Data Model Inspektor' tab in the right utilities tab.

Edit: See this screenshot