如何防止 Core Data 在 iOS 5 中重复? [英] How to prevent Core Data making duplicates in iOS 5?

问题描述

我遇到了问题.

周末我一直在做一个项目，我从网络服务中提取一个大的 xml.

Over the weekend I've been working on a project where I'm pulling a large xml from a webservice.

它基本上有 3 层 - 客户、经理、员工，都是分层的.因此，应用程序第一次运行时，它会提取此 xml 并对其进行解析并在 3 个相关实体(客户、经理和员工)中创建所有条目.

It basically has 3 tiers - Clients, Managers, Staff all hierarchical. So the first time the app runs, it pulls this xml and parses it and creates all the entries in the 3 releated Entities - Clients, Managers and Staff.

每次应用程序启动时，我都需要删除相同的 XML，但这一次，我只需要更新"任何已更改的现有记录，或为新客户、经理或员工添加新记录自上次以来出现过.

Every time the app launches I need to pull that same XML down, but this time, I only need to 'update' any of the existing records that have changed, or add new ones for new clients, managers or staff that have appeared since last time.

所以，正如我所说，目前它正在提取所有内容，正确解析并创建正确的实体并填充所有属性.

So - at the moment, as I said, it's pulling it all, parsing it correctly and creating the correct entities and filling in all the attributes.

但是，在没有数据更改的情况下，在第 2 次启动时，它会复制所有数据 - 所以我有 30 个而不是 15 个客户端(正确的数字)等等......

However, with no data change, on the 2nd launch it's DUPLICATING all of the data - so instead of 15 clients ( the correct number ) I have 30 and so on...

我真的必须在解析中添加大量代码来检查它是否已经存在，而不是创建一个新的 NSManagedObject 吗?

Do I really have to add lots of code in my parsing to check that instead of creating a new NSManagedObject, I check if it's already there?

如果是 - 我必须手动检查每个属性?

And if it is - I have to then manually check every attribute?

这非常痛苦和冗长 - 有没有办法让 Core Data 自动为我做这种事情?

That's awfully painful and longwinded - isn't there a way to make Core Data do this kinda stuff for me - automatically?

感谢您的任何帮助或建议.

Thanks for any help or suggestions.

推荐答案

如 Apple Docs 所述 https://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/CoreData/Articles/cdImporting.html

As Stated in Apple Docs https://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/CoreData/Articles/cdImporting.html

你需要循环数据模型并像这样从那里处理它

You need to loop the data model and handle it from there like this

例子:

// loop over employeeIDs
// anID = ... each employeeID in turn
// within body of loop

NSString *predicateString = [NSString stringWithFormat: @"employeeID == %@", anID];

NSPredicate *predicate = [NSPredicate predicateWithFormat:predicateString];

我个人不喜欢这种方法，我编写了这段代码，它以高效的方式处理这个问题，而且很简单！我注意到使用 Apples 方法时遇到了字符串具有不同字符(例如大写字母和空格)的问题.如果您正确地重命名所有相应的对象，下面的代码已经过测试并且可以正常工作.老实说，这是实现不在核心数据中添加重复项的最有效方法.

Personally I do not like this method and I wrote this snippet of code that handles this in a pro-efficient manor and which is straight forward! I noticed with Apples method I ran into issues with strings having different characters such as capitol letters and spaces. Below code is tested and working if you rename all your corresponding objects correctly I honestly believe this is the most efficient way to accomplish not adding duplicates in core data.

-(void)AvoidDuplicatesinDataModel
{
    // Define our table/entity to use
    NSEntityDescription *entity = [NSEntityDescription entityForName:@"Users"
                                              inManagedObjectContext:managedObjectContext];

    // Setup the fetch request
    NSFetchRequest *request = [[NSFetchRequest alloc] init];
    [request setEntity:entity];

    // Define how we will sort the records
    NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"users"
                                                                   ascending:NO];
    NSArray *sortDescriptors = [NSArray arrayWithObject:sortDescriptor];

    [request setSortDescriptors:sortDescriptors];
    [sortDescriptor release];

    // Fetch the records and handle an error
    NSError *Fetcherror;
    NSMutableArray *mutableFetchResults = [[managedObjectContext
                                            executeFetchRequest:request error:&Fetcherror] mutableCopy];

    if (!mutableFetchResults) {
        // Handle the error.
        // This is a serious error
    }

    //here usersNameTextField.text can be any (id) string that you are searching for
    if ([[mutableFetchResults valueForKey:@"users"]
         containsObject:usernameTextField.text]) {
        //Alert user or handle your duplicate methods from here
        return;
    }
}

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