如何通过其objectID获取核心数据实体？

问题描述

我有一个来自coredata的对象，然后我从这些对象之一获取objectId：

I have a list objects from coredata and then I get objectId from one of those objects:

let fetchedId = poi.objectID.URIRepresentation()

现在我需要获取这个特定objectID的实体。
我试过像：

Now I need to get entity for this specific objectID.And I tried something like:

let entityDescription = NSEntityDescription.entityForName("Person", inManagedObjectContext: managedObjectContext!);

        let request = NSFetchRequest();
        request.entity = entityDescription;

        let predicate = NSPredicate(format: "objectID = %i", fetchedId);

        request.predicate = predicate;

        var error: NSError?;

        var objects = managedObjectContext?.executeFetchRequest(request,
            error: &error)

但我收到错误：

Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'keypath objectID not found in entity <NSSQLEntity Person id=4>'

推荐答案

您无法使用NSFetchRequest的谓词查询NSManagedObject的任意属性。这只适用于在您的实体中定义的属性。

You can't query arbitrary properties of the NSManagedObject with a predicate for a NSFetchRequest. This will only work for attributes that are defined in your entity.

NSManagedObjectContext有两种方法来检索具有NSManagedObjectID的对象。如果对象在上下文中不存在，则第一个引发异常：

NSManagedObjectContext has two ways to retrieve an object with an NSManagedObjectID. The first one raises an exception if the object does not exist in the context:

managedObjectContext.objectWithID(objectID)

第二个将通过返回nil失败：

The second will fail by returning nil:

var error: NSError?
if let object = managedObjectContext.existingObjectWithID(objectID, error: &error) {
    // do something with it
}
else {
    println("Can't find object \(error)")
}

---

b $ b

如果你有一个URI而不是NSManagedObjectID，你必须先把它转换为NSManagedObjectID。 persistStoreCoordinator用于此：

---

If you have a URI instead of a NSManagedObjectID you have to turn it into a NSManagedObjectID first. The persistentStoreCoordinator is used for this:

let objectID = managedObjectContext.persistentStoreCoordinator!.managedObjectIDForURIRepresentation(uri)

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