使用按计数分组的核心数据获取属性

这是我想为 Core Data 编写的查询的 SQL 版本：

SELECT Group.Name, COUNT(Item.Name)
FROM Item INNER JOIN Group ON Item.GroupID = Group.ID
GROUP BY Group.Name

到目前为止我所拥有的是：

NSFetchRequest* fetchGroupSummary = [NSFetchRequest fetchRequestWithEntityName:@"Item"];

NSEntityDescription* entity = [NSEntityDescription entityForName:@"Item" inManagedObjectContext:[[CoreDataManager sharedInstance] managedObjectContext]];

NSAttributeDescription* groupName = [entity.relationshipsByName objectForKey:@"group"];
NSExpression *countExpression = [NSExpression expressionForFunction: @"count:" arguments: [NSArray arrayWithObject:[NSExpression expressionForKeyPath: @"name"]]];
NSExpressionDescription *expressionDescription = [[NSExpressionDescription alloc] init];

[expressionDescription setName: @"count"];
[expressionDescription setExpression: countExpression];
[expressionDescription setExpressionResultType: NSInteger32AttributeType];

NSSortDescriptor *sortDescriptor = [NSSortDescriptor sortDescriptorWithKey:@"group.sort" ascending:YES];
NSPredicate *predicate = [NSPredicate predicateWithFormat:@"group.stage == %@", stage];

[fetchGroupSummary setEntity:entity];
[fetchGroupSummary setSortDescriptors:@[sortDescriptor]];
[fetchGroupSummary setPropertiesToFetch:[NSArray arrayWithObjects:groupName, expressionDescription, nil]];
[fetchGroupSummary setPropertiesToGroupBy:[NSArray arrayWithObject:groupName]];
[fetchGroupSummary setResultType:NSDictionaryResultType];
[fetchGroupSummary setPredicate:predicate];

NSError* error = nil;
groups = [[[CoreDataManager sharedInstance] managedObjectContext] executeFetchRequest:fetchGroupSummary error:&error];

expressionDescription = nil;

这几乎给了我一切，但是我想指定 group.name 而不是 groupName 作为组关系 - 这可能吗？

Mark

setPropertiesToGroupBy of NSFetchRequest接受一个数组NSPropertyDescription or NSExpressionDescription物体，或键路径字符串。在您的情况下，您可以使用键路径字符串：

[fetchGroupSummary setPropertiesToGroupBy:[NSArray arrayWithObject:@"group.name"]];