使用Core Data检索唯一结果集 [英] Retrieving a unique result set with Core Data

问题描述

我有一个基于核心数据的应用程序来管理一堆实体。我想要能够做到以下。

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

现在，SomeEntity有几行，如果我们严格说SQL语句。我想要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。我也需要他们按照秩序返回。所以在SQL中，我要找的是以下：

SELECT DISTINCT（type）ORDER BY rank ASC

这是我到目前为止打破的代码：

  NSError * error = NULL; 
 NSFetchRequest * fetchRequest = [[NSFetchRequest alloc] init]; 
 [fetchRequest setReturnsDistinctResults：YES]; 
 [fetchRequest setPropertiesToFetch：[NSArray arrayWithObjects：@type，@rank，nil]]; 
 NSEntityDescription * entity = [NSEntityDescription entityForName：@SomeEntityinManagedObjectContext：managedObjectContext]; 
 [fetchRequest setEntity：entity]; 
 
 //按排序排序
 NSSortDescriptor * rankDescriptor = [[NSSortDescriptor alloc] initWithKey：@rankascending：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; 
  

现在使用以下代码崩溃：无效的keypath部分传递给setPropertiesToFetch ：

$

  
解决方案
 NSManagedObjectContext * ctx; / * some ctx * / 
 NSFetchRequest * req; / *你的请求* / 
 NSEntityDescription * entity = [NSEntityDescription entityForName：@SomeEntityinManagedObjectContext：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：@rankascending：YES]]]; 
 NSArray * result = [ctx executeFetchRequest：req error：nil]; 
  

I have a core data based app that manages a bunch of entities. I'm looking to be able to do the following.

I have an entity "SomeEntity" with the attributes: name, type, rank, foo1, foo2.

Now, SomeEntity has several rows if we're speaking strictly in SQL terms. What I'm trying to accomplish is to retrieve only available types, even though each instance can have duplicate types. I also need them returned in order according to rank. So in SQL, what I'm looking for is the following:

SELECT DISTINCT(type) ORDER BY rank ASC

Here is the code I have so far that's breaking:

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;

Right now that is crashing with the following: Invalid keypath section passed to setPropertiesToFetch:

解决方案

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];

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