使用核心数据检索唯一结果集

Question

我有一个基于核心数据的应用程序来管理一堆实体。我期望能够做到以下几点。

我有一个实体“SomeEntity”的属性：name，type，rank，foo1，foo2。

现在，如果我们严格按照SQL术语说话，SomeEntity有几行。我试图完成的是只检索可用的类型，即使每个实例可以有重复的类型。我也需要他们按照等级顺序归还。因此，在SQL，我正在寻找的是以下几点：

SELECT DISTINCT(type) ORDER BY rank ASC

这是我迄今多数民众赞成打破代码：与下面的崩溃

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： Invalid keypath section passed to setPropertiesToFetch:

Answer 1

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];