使用 Core Data 检索唯一结果集

我有一个基于核心数据的应用程序，用于管理一堆实体。我希望能够执行以下操作。

我有一个实体“SomeEntity”，其属性为：名称、类型、等级、foo1、foo2。

现在，如果我们严格按照 SQL 术语来讲，SomeEntity 有几行。我想要完成的是仅检索可用的类型，即使每个实例可以具有重复的类型。我还需要根据排名按顺序返回它们。所以在 SQL 中，我正在寻找以下内容：

SELECT DISTINCT(type) ORDER BY rank ASC

这是我迄今为止所破坏的代码：

NSError *error = NULL;
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
[fetchRequest setReturnsDistinctResults:YES];
[fetchRequest setPropertiesToFetch:[NSArray arrayWithObjects:@"type", @"rank", nil]];
NSEntityDescription *entity = [NSEntityDescription entityForName:@"SomeEntity" inManagedObjectContext:managedObjectContext];
[fetchRequest setEntity:entity];

// sort by rank
NSSortDescriptor *rankDescriptor = [[NSSortDescriptor alloc] initWithKey:@"rank" ascending:YES];
NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:rankDescriptor,nil];
[fetchRequest setSortDescriptors:sortDescriptors];
[sortDescriptors release];
[rankDescriptor release];

NSArray *fetchResults = [managedObjectContext executeFetchRequest:fetchRequest error:&error];

[fetchRequest release];

return fetchResults;

现在正在崩溃，原因如下：Invalid keypath section passed to setPropertiesToFetch:

NSManagedObjectContext * ctx ; /* some ctx */    
NSFetchRequest * req; /* your request */
NSEntityDescription *entity = [NSEntityDescription entityForName:@"SomeEntity" inManagedObjectContext:context];
NSDictionary *entityProperties = [entity propertiesByName];
[req setEntity:entity];
[req setReturnsDistinctResults:YES];
[req setPropertiesToFetch:[NSArray arrayWithObject:[entityProperties objectForKey:@"type"]]];
[req setSortDescriptors:[NSArray arrayWithObject:[[NSSortDescriptor alloc] initWithKey:@"rank" ascending:YES]]];
NSArray * result = [ctx executeFetchRequest:req error:nil];