使用核心數據檢索唯一結果集

Question

我有一個基於核心數據的應用程序來管理一堆實體。我期望能夠做到以下幾點。

我有一個實體「SomeEntity」的屬性：name，type，rank，foo1，foo2。

現在，如果我們嚴格按照SQL術語說話，SomeEntity有幾行。我試圖完成的是隻檢索可用的類型，即使每個實例可以有重複的類型。我也需要他們按照等級順序歸還。因此，在SQL，我正在尋找的是以下幾點：

SELECT DISTINCT(type) ORDER BY rank ASC

這是我迄今多數民衆贊成打破代碼：與下面的崩潰

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; 

現在： 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];