Rdzeń danych Pobieranie Properties z grupą przez hrabiego

Question

To jest wersja SQL kwerendy Chciałbym napisać dla danych Core:

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

tej pory co mam to:

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; 

ten prawie daje mi wszystko, jednak zamiast groupName będącego relacją grupy chciałbym podać group.name - czy to możliwe?

Mark

Answer 1

setPropertiesToGroupBy z NSFetchRequest akceptuje tablicę NSPropertyDescription lub NSExpressionDescription obiektów lub keypath łańcuchów. W twoim przypadku można użyć ciągu keypath:

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