Exception thrown in NSOrderedSet generated accessors

I reproduced your setup both with your data model and one of my own with different names. I got the same error in both cases.

Looks like a bug in Apple's autogenerated code.

I agree that there may be a bug here. I've modified the implementation of the add object setter to append correctly to a NSMutableOrderedSet.

- (void)addSubitemsObject:(SubItem *)value {
    NSMutableOrderedSet* tempSet = [NSMutableOrderedSet orderedSetWithOrderedSet:self.subitems];
    [tempSet addObject:value];
    self.subitems = tempSet;
}

Reassigning the set to self.subitems will ensure that the Will/DidChangeValue notifications are sent.

I've decided to improve the solution by implementing all the required methods:

static NSString *const kItemsKey = @"<#property#>";

- (void)insertObject:(<#Type#> *)value in<#Property#>AtIndex:(NSUInteger)idx {
    NSIndexSet* indexes = [NSIndexSet indexSetWithIndex:idx];
    [self willChange:NSKeyValueChangeInsertion valuesAtIndexes:indexes forKey:kItemsKey];
    NSMutableOrderedSet *tmpOrderedSet = [NSMutableOrderedSet orderedSetWithOrderedSet:[self mutableOrderedSetValueForKey:kItemsKey]];
    [tmpOrderedSet insertObject:value atIndex:idx];
    [self setPrimitiveValue:tmpOrderedSet forKey:kItemsKey];
    [self didChange:NSKeyValueChangeInsertion valuesAtIndexes:indexes forKey:kItemsKey];
}

- (void)removeObjectFrom<#Property#>AtIndex:(NSUInteger)idx {
    NSIndexSet* indexes = [NSIndexSet indexSetWithIndex:idx];
    [self willChange:NSKeyValueChangeRemoval valuesAtIndexes:indexes forKey:kItemsKey];
    NSMutableOrderedSet *tmpOrderedSet = [NSMutableOrderedSet orderedSetWithOrderedSet:[self mutableOrderedSetValueForKey:kItemsKey]];
    [tmpOrderedSet removeObjectAtIndex:idx];
    [self setPrimitiveValue:tmpOrderedSet forKey:kItemsKey];
    [self didChange:NSKeyValueChangeRemoval valuesAtIndexes:indexes forKey:kItemsKey];
}

- (void)insert<#Property#>:(NSArray *)values atIndexes:(NSIndexSet *)indexes {
    [self willChange:NSKeyValueChangeInsertion valuesAtIndexes:indexes forKey:kItemsKey];
    NSMutableOrderedSet *tmpOrderedSet = [NSMutableOrderedSet orderedSetWithOrderedSet:[self mutableOrderedSetValueForKey:kItemsKey]];
    [tmpOrderedSet insertObjects:values atIndexes:indexes];
    [self setPrimitiveValue:tmpOrderedSet forKey:kItemsKey];
    [self didChange:NSKeyValueChangeInsertion valuesAtIndexes:indexes forKey:kItemsKey];
}

- (void)remove<#Property#>AtIndexes:(NSIndexSet *)indexes {
    [self willChange:NSKeyValueChangeRemoval valuesAtIndexes:indexes forKey:kItemsKey];
    NSMutableOrderedSet *tmpOrderedSet = [NSMutableOrderedSet orderedSetWithOrderedSet:[self mutableOrderedSetValueForKey:kItemsKey]];
    [tmpOrderedSet removeObjectsAtIndexes:indexes];
    [self setPrimitiveValue:tmpOrderedSet forKey:kItemsKey];
    [self didChange:NSKeyValueChangeRemoval valuesAtIndexes:indexes forKey:kItemsKey];
}

- (void)replaceObjectIn<#Property#>AtIndex:(NSUInteger)idx withObject:(<#Type#> *)value {
    NSIndexSet* indexes = [NSIndexSet indexSetWithIndex:idx];
    [self willChange:NSKeyValueChangeReplacement valuesAtIndexes:indexes forKey:kItemsKey];
    NSMutableOrderedSet *tmpOrderedSet = [NSMutableOrderedSet orderedSetWithOrderedSet:[self mutableOrderedSetValueForKey:kItemsKey]];
    [tmpOrderedSet replaceObjectAtIndex:idx withObject:value];
    [self setPrimitiveValue:tmpOrderedSet forKey:kItemsKey];
    [self didChange:NSKeyValueChangeReplacement valuesAtIndexes:indexes forKey:kItemsKey];
}

- (void)replace<#Property#>AtIndexes:(NSIndexSet *)indexes with<#Property#>:(NSArray *)values {
    [self willChange:NSKeyValueChangeReplacement valuesAtIndexes:indexes forKey:kItemsKey];
    NSMutableOrderedSet *tmpOrderedSet = [NSMutableOrderedSet orderedSetWithOrderedSet:[self mutableOrderedSetValueForKey:kItemsKey]];
    [tmpOrderedSet replaceObjectsAtIndexes:indexes withObjects:values];
    [self setPrimitiveValue:tmpOrderedSet forKey:kItemsKey];
    [self didChange:NSKeyValueChangeReplacement valuesAtIndexes:indexes forKey:kItemsKey];
}

- (void)add<#Property#>Object:(<#Type#> *)value {
    NSMutableOrderedSet *tmpOrderedSet = [NSMutableOrderedSet orderedSetWithOrderedSet:[self mutableOrderedSetValueForKey:kItemsKey]];
    NSUInteger idx = [tmpOrderedSet count];
    NSIndexSet* indexes = [NSIndexSet indexSetWithIndex:idx];
    [self willChange:NSKeyValueChangeInsertion valuesAtIndexes:indexes forKey:kItemsKey];
    [tmpOrderedSet addObject:value];
    [self setPrimitiveValue:tmpOrderedSet forKey:kItemsKey];
    [self didChange:NSKeyValueChangeInsertion valuesAtIndexes:indexes forKey:kItemsKey];
}

- (void)remove<#Property#>Object:(<#Type#> *)value {
    NSMutableOrderedSet *tmpOrderedSet = [NSMutableOrderedSet orderedSetWithOrderedSet:[self mutableOrderedSetValueForKey:kItemsKey]];
    NSUInteger idx = [tmpOrderedSet indexOfObject:value];
    if (idx != NSNotFound) {
        NSIndexSet* indexes = [NSIndexSet indexSetWithIndex:idx];
        [self willChange:NSKeyValueChangeRemoval valuesAtIndexes:indexes forKey:kItemsKey];
        [tmpOrderedSet removeObject:value];
        [self setPrimitiveValue:tmpOrderedSet forKey:kItemsKey];
        [self didChange:NSKeyValueChangeRemoval valuesAtIndexes:indexes forKey:kItemsKey];
    }
}

- (void)add<#Property#>:(NSOrderedSet *)values {
    NSMutableOrderedSet *tmpOrderedSet = [NSMutableOrderedSet orderedSetWithOrderedSet:[self mutableOrderedSetValueForKey:kItemsKey]];
    NSMutableIndexSet *indexes = [NSMutableIndexSet indexSet];
    NSUInteger valuesCount = [values count];
    NSUInteger objectsCount = [tmpOrderedSet count];
    for (NSUInteger i = 0; i < valuesCount; ++i) {
        [indexes addIndex:(objectsCount + i)];
    }
    if (valuesCount > 0) {
        [self willChange:NSKeyValueChangeInsertion valuesAtIndexes:indexes forKey:kItemsKey];
        [tmpOrderedSet addObjectsFromArray:[values array]];
        [self setPrimitiveValue:tmpOrderedSet forKey:kItemsKey];
        [self didChange:NSKeyValueChangeInsertion valuesAtIndexes:indexes forKey:kItemsKey];
    }
}

- (void)remove<#Property#>:(NSOrderedSet *)values {
    NSMutableOrderedSet *tmpOrderedSet = [NSMutableOrderedSet orderedSetWithOrderedSet:[self mutableOrderedSetValueForKey:kItemsKey]];
    NSMutableIndexSet *indexes = [NSMutableIndexSet indexSet];
    for (id value in values) {
        NSUInteger idx = [tmpOrderedSet indexOfObject:value];
        if (idx != NSNotFound) {
            [indexes addIndex:idx];
        }
    }
    if ([indexes count] > 0) {
        [self willChange:NSKeyValueChangeRemoval valuesAtIndexes:indexes forKey:kItemsKey];
        [tmpOrderedSet removeObjectsAtIndexes:indexes];
        [self setPrimitiveValue:tmpOrderedSet forKey:kItemsKey];
        [self didChange:NSKeyValueChangeRemoval valuesAtIndexes:indexes forKey:kItemsKey];
    }
}

Yes, this is definitely a Core Data bug. I wrote up an ObjC-Runtime-based fix a while back, but at the time I figured it would be fixed soon. Anyway, no such luck, so I posted it up on GitHub as KCOrderedAccessorFix. Work around the problem on all your entities:

[managedObjectModel kc_generateOrderedSetAccessors];

One entity in particular:

[managedObjectModel kc_generateOrderedSetAccessorsForEntity:entity];

Or just for one relationship:

[managedObjectModel kc_generateOrderedSetAccessorsForRelationship:relationship];

Instead to making a copy I suggest to use the accessor in NSObject to get access to the NSMutableOrderedSet of the relationships.

- (void)addSubitemsObject:(SubItem *)value {
      NSMutableOrderedSet* tempSet = [self mutableOrderedSetValueForKey:@"subitems"];
      [tempSet addObject:value];
 }

e.g. the Core Data Release Notes for iOS v5.0 refer to this.

In a short test it worked in my application.