僅有一行的Coredata實體

Question

我的coredata表之一（呃，實體）應該只存儲一行數據。 如果該行不存在，則應創建該行，如果該行已經存在，則應使用新數據覆蓋（或編輯）同一行。

當前，在我的實現中，總是將新行添加到實體（名為“ TempNames”）：

/* Store names data in temporary name table. */
TempNames *tempNames = [NSEntityDescription insertNewObjectForEntityForName:@"TempNames" inManagedObjectContext:context];
tempNames.namesData = tempNamesData;

誰能給我一些提示，將其更改為所需的功能需要什么？ 我想需要NSPredicate才能實現我想要的？

---

更新的工作實現：

/* Convert names array into serializable data. */
NSData *tempNamesData = [NSKeyedArchiver archivedDataWithRootObject:names];

/* Store names data in temporary name table. */
TempNames *tempNames = nil;
NSError *error;
NSFetchRequest *req = [NSFetchRequest fetchRequestWithEntityName:@"TempNames"];
NSArray *records = [context executeFetchRequest:req error:&error];

/* Record already exists. */
if (records.count > 0)
{
    tempNames = records.firstObject;
}
else
{
    tempNames = [NSEntityDescription insertNewObjectForEntityForName:@"TempNames" inManagedObjectContext:context];
}
tempNames.namesData = tempNamesData;

[context save:&error];

Answer 1

像這樣嘗試：

TempNames *tempNames = nil;
NSFetchRequest *appRequest = [NSFetchRequest fetchRequestWithEntityName:@"TempNames"];
    NSArray *allNames = [context executeFetchRequest:appRequest error:nil];

if (allNames.count > 0) {
   // your record exists
   tempNames = allNames.firstObject;
} else {
     tempNames = [NSEntityDescription insertNewObjectForEntityForName:@"TempNames" inManagedObjectContext:context];
}
tempNames.namesData = tempNamesData;

Answer 2

如果每次都插入一個新實體，則不會執行插入/更新操作，即使該實體的數據相同，它也始終是插入操作。 您應該做的是首先獲取是否有存儲中的數據，然后確定是否需要插入或更新數據。

NSFetchRequest *request = [NSFetchRequest fetchRequestWithEntityName:@"TempNames"];
NSError *error;
NSArray *entities = [context executeFetchRequest:request error:&error];
if (entities.count > 0) {
    // You have already inserted the entity
}
else {
    // Insert your new entity
    ...
    // Save changes to the store
    NSError *error;
    [context save:&error];
}