CoreData获取不同的属性值 [英] CoreData get distinct values of Attribute

问题描述

我试图将 NSFetchRequest 设置为核心数据，以检索实体中特定属性的唯一值。即

I'm trying to setup my NSFetchRequest to core data to retrieve the unique values for a specific attribute in an entity. i.e.

具有以下信息的实体：

  name | rate | factor |
_______|______|________|
John   |  3.2 |    4   |
Betty  |  5.5 |    7   |
Betty  |  2.1 |    2   |
Betty  |  3.1 |    2   |
Edward |  4.5 |    5   |
John   |  2.3 |    4   |

我如何设置返回一个数组的请求：John，Betty，Edward？

How would i set up the request to return an array with just: John, Betty, Edward?

推荐答案

您应该使用后备存储来帮助您获得不同的记录。

You should use the backing store to help you get distinct records.

如果你想得到一个数组只有约翰，贝蒂，爱德华，这里是如何做：

If you want to get an array with just John, Betty, Edward here's how you do it:

NSFetchRequest *fetchRequest = [NSFetchRequest fetchRequestWithEntityName:@"MyEntity"];
NSEntityDescription *entity = [NSEntityDescription entityForName:@"MyEntity" inManagedObjectContext:self.managedObjectContext];

// Required! Unless you set the resultType to NSDictionaryResultType, distinct can't work. 
// All objects in the backing store are implicitly distinct, but two dictionaries can be duplicates.
// Since you only want distinct names, only ask for the 'name' property.
fetchRequest.resultType = NSDictionaryResultType;
fetchRequest.propertiesToFetch = [NSArray arrayWithObject:[[entity propertiesByName] objectForKey:@"name"]];
fetchRequest.returnsDistinctResults = YES;

// Now it should yield an NSArray of distinct values in dictionaries.
NSArray *dictionaries = [self.managedObjectContext executeFetchRequest:fetchRequest error:nil];
NSLog (@"names: %@",dictionaries);

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