如何检查核心数据中是否存在对象

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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