核心数据获取属性（按计数）

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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