CoreData限制相关实体的批次大小

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程序，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.

Thx提供帮助。

推荐答案

提取请求始终从托管对象上下文返回对象。您无法获取已修改的对象，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.

但是您可以做的是取与给定用户的与
相关的 CoreMessage 对象：

But what you can do instead is fetch the CoreMessage objects relatedto the given user:

let fetchRequest = NSFetchRequest(entityName: "CoreMessage")
fetchRequest.predicate = NSPredicate(format: "owner.id == %@", nodeId)

现在，您可以添加抓取限制以限制检索到的
消息的数量：

and now you can add a fetch limit to restrict the number of retrievedmessages:

fetchRequest.fetchLimit = 20

连同合适的排序描述符。

together with a suitable sort descriptor.

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