核心数据获取属性(按计数) [英] Core Data Fetching Properties with Group By Count
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问题描述
这是我要为Core Data写入的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,我有:
NSFetchRequest * fetchGroupSummary = [NSFetchRequest fetchRequestWithEntityName:@ 项目];
NSEntityDescription * entity = [NSEntityDescription entityForName:@IteminManagedObjectContext:[[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.sortascending: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错误:&错误];
expressionDescription = nil;
这几乎给了我一切,但是不是groupName是组关系,我想指定组。
解决方案
setPropertiesToGroupBy of
NSFetchRequest
接受 NSPropertyDescription
或 NSExpressionDescription
对象或 keypath字符串。在您的情况下,您可以使用键盘字符串:
[fetchGroupSummary setPropertiesToGroupBy:[NSArray arrayWithObject:@group.name] ];
This is the SQL version of the query I would like to write for Core Data:
SELECT Group.Name, COUNT(Item.Name)
FROM Item INNER JOIN Group ON Item.GroupID = Group.ID
GROUP BY Group.Name
So far what I have is:
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;
This almost gives me everything, however instead of groupName being the group relationship I would like to specify group.name - is this possible?
Mark
解决方案
setPropertiesToGroupBy
of NSFetchRequest
accepts an array of NSPropertyDescription
or NSExpressionDescription
objects, or keypath strings. In your case, you can use a keypath string:
[fetchGroupSummary setPropertiesToGroupBy:[NSArray arrayWithObject:@"group.name"]];
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