为什么这个代码提高“CoreData：error：（19）PRIMARY KEY必须是唯一的”错误？

问题描述

此代码引发CoreData：错误：（19）PRIMARY KEY必须是唯一的错误。
日实体只有当属性是 NSDate ，以及一个名为 tasks 的一对多关系。为什么会出现这个错误？如果已经存储了具有特定日期的 Day ，则会提取它，否则我将其插入。因此，对于每一天的对象，当属性时应该有不同的。我不知道这是否是主键虽然。如何解决这个问题？先谢谢你。

This code raises the "CoreData: error: (19) PRIMARY KEY must be unique" error.The Day entity has only a when attribute which is an NSDate, and a to-many relationship called tasks. Why this error? If a Day with a specific date is already stored, I fetch it, otherwise I insert it. So, for each day object, there should be a different when attribute. I am not sure if this is the primary key though. How to solve this ? Thank you in advance.

   NSMutableSet *occurrences = nil;

   occurrences = ... 
   NSMutableOrderedSet *newSet = [NSMutableOrderedSet orderedSetWithCapacity:[occurrences count]];

   for(NSDate *current in occurrences) {

        NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];

        // try to find a corresponding Day entity whose when attribute is equal to the current occurrence
        // if none is available, create it

        Day * day = [[self getDayForDate:current inManagedObjectContext:moc] retain];
        if(!day){
            day = (Day *) [NSEntityDescription insertNewObjectForEntityForName:@"Day" inManagedObjectContext:moc];
        }

        day.when = current;
        [day addTasksObject:aTask];

        [newSet addObject:day];

        [moc insertObject:day];
        [moc processPendingChanges];

        [day release];

        [pool release];            
    } 

- (Day *)getDayForDate:(NSDate *)aDate inManagedObjectContext:(NSManagedObjectContext *)moc
{        
    NSFetchRequest *request = [[NSFetchRequest alloc] init];
    NSEntityDescription *entity = [NSEntityDescription entityForName:@"Day" inManagedObjectContext:moc];
    [request setEntity:entity];
    NSPredicate *predicate = [NSPredicate predicateWithFormat:@"(when == %@)", aDate];
    [request setPredicate:predicate];
    NSError *error = nil;
    NSArray *array = [moc executeFetchRequest:request error:&error];
    [request release];

    Day *theDay = nil;

    if(array && [array count] == 1){
        theDay = [array objectAtIndex:0];
    }

    return theDay;
}

推荐答案

如果你已经有了，就需要插入一个新的 Day （这是day不是nil的情况）。特别是我指的是 [moc insertObject：day] 。

I guess you don't need to insert a new Day if you already have it (this is the case when day is not nil). In particular I'm referring to [moc insertObject:day].

如果使用 insertNewObjectForEntityForName ，当您保存moc时，该方法为您插入对象。如果你需要修改它（你已经检索到一个非零的一天）修改它并保存。另外，当循环完成（仅仅是为了性能原因）时，我会做 processPendingChanges 。

If you use insertNewObjectForEntityForName, that method inserts the object for you when you save the moc. If you need to modify it (you have retrieved a non-nil day) modify it and save. In additon, I will do processPendingChanges when the loop finishes (just for performance reasons).

希望有帮助。

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