Question

我想要做到的是: 我需要将核心数据条目的数量限制在50个。因此,如果用户进入第50条条目,则该用户将删除最老的条目,并将新条目添加到顶端。因此,如果用户永远不删除条目,如果核心数据有50个条目,那么当用户试图增加新的条目时,则该用户将删除最老的条目,并增加用户的新条目。基本上,我试图有历史背景,但我不想让用户能够接过50个条目,然而,我希望他们在50个限制中能够增加新的条目,仅仅放弃最古老的条目,并增加最新的条目。这样做的最容易的方法是什么? I m new to core data and with augh time Understanding a Much of it. 这里的法典/实例表明,我与我合作。附录一得到帮助。

Answer 1

请允许我说,有一个名为<代码>History的实体。最容易的解决办法是添加一个<条码>,/code>,归属于贵实体。然后使用<代码>History物体。

你们需要三个方面:

The first one will fetch as faults all the existing History objects and then count them. If the count is <50, then just add the new History object and your done.

If the count>=50, then do a fetch for specific value and use the @max or @min (I forget which for dates) collections operator to find the oldest creationDate. (As luck would have it the example at the link it pretty much exactly what you need.)
Perform a fetch for the object with the creationDate returned by (2) and delete it.

然后添加新的历史目标。

Answer 2

缩略语核心捐款不会给你,但你可以这样做。

You can retrieve objects from you context using an NSFetchRequest, and you can delete them using -[NSManagedObjectContext deleteObject:]. You can sort them using NSSortDescriptor objects.

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