'对多关键在这里不允许'例外，但为什么？

Question

我建立一个地图应用有以下实体：

协调，地图是对一个地图坐标是一个一对多的关系。

而且我的代码是：

fileprivate var fetchedResultsControllerForMapEntity: NSFetchedResultsController<Map> = { 
// Create Fetch Request 
let fetchRequest: NSFetchRequest<Map> = Map.fetchRequest() 

// Configure Fetch Request 
fetchRequest.sortDescriptors = [NSSortDescriptor(key: "name", ascending: true)] 
fetchRequest.predicate = NSPredicate(format:"newRelationship == %@", "peachesfarm") 

// Create Fetched Results Controller 
let fetchedResultsControllerForMapEntity = NSFetchedResultsController(fetchRequest: fetchRequest, managedObjectContext: managedObjectContext, sectionNameKeyPath: nil, cacheName: nil) 

return fetchedResultsControllerForMapEntity 
}() 

fileprivate var fetchedResultsControllerForCoordinateEntity: NSFetchedResultsController<Coordinate> = { 
// Create Fetch Request 
let fetchRequest: NSFetchRequest<Coordinate> = Coordinate.fetchRequest() 

// Configure Fetch Request 
fetchRequest.sortDescriptors = [NSSortDescriptor(key: "latitude", ascending: true)] 
fetchRequest.sortDescriptors = [NSSortDescriptor(key: "longitude", ascending: true)] 

// Create Fetched Results Controller 
let fetchedResultsControllerForCoordinateEntity = NSFetchedResultsController(fetchRequest: fetchRequest, managedObjectContext: managedObjectContext, sectionNameKeyPath: nil, cacheName: nil) 

return fetchedResultsControllerForCoordinateEntity 
}() 

我基本上是试图检索名为“peachesfarm”只有地图的坐标。为什么我会得到这个异常？

Answer 1

NSPredicate(format:"newRelationship == %@", "peachesfarm") 

给出了一个错误，因为“newRelationship”是一个一对多的关系，这 不能平等进行比较。

要获取与给定名称的地图相关的所有坐标， 必须使用“坐标”的获取请求，而不是“地图”。 然后使用关键路径指向相关地图的名称。使用 关系名称由@vadian如上文所建议的将是

let fetchRequest: NSFetchRequest<Coordinate> = Coordinate.fetchRequest() 
let predicate = NSPredicate(format: "map.name == %@", "peachesFarm") 
fetchRequest.predicate = predicate 

更妙的是，使用#keyPath指令，在编译器检查 属性名称，从而避免错误：

let predicate = NSPredicate(format: "\(#keyPath(Coordinate.map.name)) == %@", "peachesFarm")