处理核心数据中的重复条目 [英] Handling duplicate entries in Core Data

问题描述

我有一个允许用户保存收藏夹的应用程序.我正在使用Core Data将收藏夹存储为托管对象.我已经编写了一些代码来防止存储重复项的可能性，但是我想知道是否有更好的方法来存储重复项.每个收藏夹对象都有一个唯一的ID字段.在下面的代码中，我只是循环遍历并检查ID字段，如果该值已经存在，请将标志值设置为true，然后退出循环.

I have an app that allows users to save favorites. I am using Core Data to store the favorites as managed objects. I have written some code to prevent the possibility of storing duplicates, but am wondering if there is a better way to do so. Each favorite object has an ID field that is unique. In the following code I am simply looping through and checking the ID field, and if the value already exists, setting a flag value to true, and breaking out of the loop.

-(BOOL)addFavorite{
    BOOL entityExists = NO;
    if(context){
        // does this favorite already exist?
        NSArray *allFaves = [AppDataAccess getAllFavorites];
        for(Favorite *f in allFaves){
            if([f.stationIdentifier isEqualToString:stID]){
                entityExists = YES;
                break;
            }
        }
        if(!entityExists){
            NSError *err = nil;
            Favorite *fave = [Favorite insertInManagedObjectContext:context];
            fave.stationRealName = riverGauge.name;
            fave.stationIdentifier = stID;
            fave.stationState = @"WV";
            if(![context save:&err]){
                NSLog(@"ERROR: Could not save context--%@", err);
            }
            return YES;            
        }
    return NO;
}

我想知道Core Data是否能够检查要添加的对象是否重复.有谓词可以处理重复项检查吗?谢谢！

I was wondering if Core Data has the ability to check to see if an object being added is a duplicate. Is there a predicate that can handle checking for duplicates? Thanks!

推荐答案

CoreData本身不会唯一.它没有两个条目相同的概念.

CoreData does no uniquing by itself. It has no notion of two entries being identical.

要启用这种行为，您必须自己执行插入前搜索"或创建前提取"来自己实现.

To enable such a behavior you have to implement it yourself by doing a 'search before insert' aka a 'fetch before create'.

NSFetchRequest *fetch = [NSFetchRequest fetchRequestWithEntityName:@"Favorite"];
NSPredicate *predicate = [NSPredicate predicateWithFormat:@"stationIdentifier == %@", stID];
[fetch setPredicate:predicate];
YourObject *obj = [ctx executeRequest:fetch];

if(!obj) {
    //not there so create it and save
    obj = [ctx insertNewManagedObjectForEntity:@"Favorite"]; //typed inline, dont know actual method
    obj.stationIdentifier = stID;
    [ctx save];
}

//use obj... e.g.
NSLog(@"%@", obj.stationIdentifier);

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