ios 如何使用谓词从 Coredata 中获取单个对象
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How to fetch a single object from Coredata using a predicate
提问by HurkNburkS
I am trying to fetch a single object from my coredatabase, however it keeps returning null. My method is based off another method which returns every value from the coredata object that I am accessing..
我试图从我的核心数据库中获取单个对象，但它一直返回 null。我的方法基于另一种方法，该方法返回我正在访问的 coredata 对象中的每个值。
I have never tried this before and have tried reading apples documents but its just not making sense.. this is what my method looks like
我以前从未尝试过，也曾尝试阅读苹果文档，但它只是没有意义……这就是我的方法的样子
- (NSMutableArray *)readSelectedInstall:(NSString *)projIDString {
NSManagedObjectContext *context = [self managedObjectContext];
if (context == nil) {
NSLog(@"Nil");
}
else {
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:@"InstallProject" inManagedObjectContext:context];
[fetchRequest setEntity:entity];
NSPredicate *predicate = [NSPredicate predicateWithFormat:@"ProjID==%@", projIDString];
[fetchRequest setPredicate:predicate];
NSError *error;
NSMutableArray *installProjectDictionaryArray = [[NSMutableArray alloc] init];
NSArray *fetchedObjects = [context executeFetchRequest:fetchRequest error:&error];
for (InstallProject *installProj in fetchedObjects) {
NSMutableDictionary *tempInstallProjectDictionaryArray = [[ NSMutableDictionary alloc] init];
[tempInstallProjectDictionaryArray setObject:installProj.companyName forKey:@"CompanyName"];
[tempInstallProjectDictionaryArray setObject:installProj.projNo forKey:@"ProjNo"];
[tempInstallProjectDictionaryArray setObject:installProj.projID forKey:@"ProjID"];
[installProjectDictionaryArray addObject:tempInstallProjectDictionaryArray];
}
return installProjectDictionaryArray;
}
return nil;
}
any help getting me to return a single item thats projID matches the projIDString would be greatly appreciated.
任何让我返回 projID 与 projIDString 匹配的单个项目的帮助将不胜感激。
回答by karthika
Import NSManagedObject(InstallProject) and fetch one object like this,
导入 NSManagedObject(InstallProject) 并像这样获取一个对象，
-(InstallProject *)readSelectedInstall:(NSString *)projIDString
{
NSArray *fetchedObjects;
NSManagedObjectContext *context = [self managedObjectContext];
NSFetchRequest *fetch = [[NSFetchRequest alloc] init];
NSEntityDescription *entityDescription = [NSEntityDescription entityForName:@"InstallProject" inManagedObjectContext: context];
[fetch setEntity:entityDescription];
[fetch setPredicate:[NSPredicate predicateWithFormat:@"(ANY ProjID contains[cd] %@)",projIDString]];
NSError * error = nil;
fetchedObjects = [context executeFetchRequest:fetch error:&error];
if([fetchedObjects count] == 1)
return [fetchedObjects objectAtIndex:0];
else
return nil;
}
回答by JohnVanDijk
What about the fetchLimiton the fetchRequestYou can set that to 1
关于什么fetchLimit的fetchRequest您可以设置到1
The fetch limit specifies the maximum number of objects that a request should return when executed.
获取限制指定请求在执行时应返回的最大对象数。
Eg. In Swift :
例如。在斯威夫特：
let fetchRequest = NSFetchRequest(entityName: "Cart")
fetchRequest.fetchLimit = 1
fetchRequest.sortDescriptors = [NSSortDescriptor(key: "creationDate", ascending: true)]
Edit:The sortDescriptors are set only because I was using this fetchRequest with an NFRC, you can just execute the fetchRequest too :
编辑：sortDescriptors 的设置只是因为我将这个 fetchRequest 与 NFRC 一起使用，您也可以执行 fetchRequest ：
do {
let fetchRequest = NSFetchRequest(entityName: "Cart")
fetchRequest.fetchLimit = 1
fetchRequest.predicate = NSPredicate(format: "name == %@", mainCart)
var objects: [Cart]
try objects = mainMoc.executeFetchRequest(fetchRequest) as! [Cart]
} catch {
}
