如何检查核心数据中是否存在对象 [英] How to check if object exists in core data

问题描述

我在这里搜索了答案，但没有一个解决我的问题...

我已经保存了一些数据在核心数据，在卡车托管对象子类。这里是代码：


I've searched answer here, but none of these solved my problem...
I've saved some data in core data, in Truck Managed Object Subclass. Here's the code:

for (NSDictionary *dictTruck in self.trucksArray) {
        Truck *truck = [NSEntityDescription insertNewObjectForEntityForName:@"Truck" inManagedObjectContext:_managedObjectContext];
        Country *country = [NSEntityDescription insertNewObjectForEntityForName:@"Country" inManagedObjectContext:_managedObjectContext];

        NSFetchRequest *truckRequest = [[NSFetchRequest alloc]init];
        NSEntityDescription *truckDescription = [NSEntityDescription entityForName:@"Truck" inManagedObjectContext:_managedObjectContext];
        [truckRequest setEntity:truckDescription];
        NSMutableArray *mutableReq = [[_managedObjectContext executeFetchRequest:truckRequest error:nil]mutableCopy];
        self.trucksArray = [[NSMutableArray alloc]initWithArray:mutableReq];
        [truck setModel:[dictTruck objectForKey:@"model"]];
        [truck setYear:[dictTruck objectForKey:@"year"]];
        [truck setCountry:country];
        [country setCountryName:[dictTruck objectForKey:@"country"]];
        [_managedObjectContext save:nil];

    }

。现在，问题是我有JSON，我从服务器通过 AFNetworking 以这样的格式下载：

And this works pretty fine. Now, problem is that I have JSON that I'm downloading from server via AFNetworking in format like this:

{
    "trucks": [
        {
            "model": "Scania",
            "year": "2014",
            "country": "Sweden"
        },
        {
            "model": "DAF",
            "year": "2012",
            "country": "Ireland"
        }
    ]
}

这个JSON每小时更新，所以有旧的JSON数据+新...现在，我的问题是如何检查这个对象是否存在于Core Data中，例如：Truck.model = @Scania，Truck.year = @2014，Truck.Country.countryName = @瑞典;如果它匹配所有这些条件，它不应该保存在CD ...

提前感谢。

更新：
在此代码中是核心数据中对象的请求，因为我已经尝试过很多方法来解决这个问题。

推荐答案

在我看来，在应用程序中应该有 UNIQUE truck_id 响应json，像这样

In my opinion there should be a UNIQUE truck_id in the response json, like this

{
    "trucks": [
        {
            "id": "00000001"
            "model": "Scania",
            "year": "2014",
            "country": "Sweden"
        },
        {
            "id": "00000002"
            "model": "DAF",
            "year": "2012",
            "country": "Ireland"
        }
    ]
}

保存 id 到到模型，并且每次从服务器获取响应时，只需检查是否存在具有相同 id 。

Save the id to Truck model and every time you get response from server, just check if there is a model with same id in core data.

对于每个卡车json，首先提出获取请求

For every truck json, make a fetch request first

NSFetchRequest *request = [[[NSFetchRequest alloc] init] autorelease];
request.entity = [NSEntityDescription entityForName:@"Truck" inManagedObjectContext:context];
NSPredicate *predicate = [NSPredicate predicateWithFormat:@"truckID = %d", truckIDInResponse];
request.predicate = predicate;
NSError *error = nil;
NSArray *objs = [context executeFetchRequest:request error:&error];
if (error) {
    [NSException raise:@"no truck find" format:@"%@", [error localizedDescription]];
}
if (objs.count > 0) {
    // there is a truck with same id exsist. Use update method
}else {
    // there's no truck with same id. Use insert method
}

truckID 是卡车模型的属性。

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