CoreData 限制相关实体批量大小 [英] CoreData limit related entities batch size
问题描述
我遇到了一个非常简单的 CoreData 问题,但似乎很难解决.编写一个简单的聊天应用,每个用户都有很多消息.
I ran into a really simple CoreData issue but it seems hard to solve. Writing a simple chat app where every user has many messages.
class CoreUser: NSManagedObject
{
@NSManaged var firstname: String?
@NSManaged var id: String?
@NSManaged var lastname: String?
@NSManaged var nickname: String?
@NSManaged var messages: NSSet
}
class CoreMessage: NSManagedObject
{
@NSManaged var body: String?
@NSManaged var owner: CoreUser
@NSManaged var state: String
@NSManaged var timestamp: NSDate
}
关系是:一对多
获取用户实体:
func getCoreUser(userId: String) -> CoreUser?
{
var error: NSError?;
let fetchRequest = NSFetchRequest(entityName: "CoreUser");
let nodeId = NSString(string: userId.lowercaseString);
let pred = NSPredicate(format: "id == %@", nodeId);
fetchRequest.fetchLimit = 1;
fetchRequest.predicate = pred;
let fetchResults = self.managedObjectContext!.executeFetchRequest(fetchRequest, error: &error) as? [CoreUser];
if let results = fetchResults
{
return results.first;
}
return nil;
}
如何限制相关消息实体(20 条消息,按日期排序).结果数组包含来自该用户的所有消息.
How can I limit the related message entity (20 messages, sorted by date). The result array contains all the messages from this user.
感谢您的帮助.
推荐答案
获取请求始终从托管对象上下文中返回对象.您无法获取修改过的"对象,例如CoreUser
对象与 CoreMessage
具有受限"关系.
A fetch request always returns the objects from the managed object context. You cannot fetch "modified" objects, e.g. CoreUser
objects with a "restricted" relationship to CoreMessage
.
但是你可以做的是获取相关的 CoreMessage
对象给给定的用户:
But what you can do instead is fetch the CoreMessage
objects related
to the given user:
let fetchRequest = NSFetchRequest(entityName: "CoreMessage")
fetchRequest.predicate = NSPredicate(format: "owner.id == %@", nodeId)
现在您可以添加提取限制来限制检索的数量消息:
and now you can add a fetch limit to restrict the number of retrieved messages:
fetchRequest.fetchLimit = 20
连同合适的排序描述符.
together with a suitable sort descriptor.
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