Retrieve Core Data Entities in Order of Insertion

问题

Is there an internal ID variable or timestamp I can use in a NSSortDescriptor to retrieve Core Data entities in the order they were inserted?

I would rather not have to create such properties if there is a cleaner way, though will obviously do so if there are no other alternatives.

回答1:

No, if you want a sort order for your data, you need to add that to your entities yourself. However you can easily add a NSDate attribute that has a default value of "NOW" so that it will auto-populate.

回答2:

You can also add an attribute to your entity, name it something like "sortNum", and whenever you fetch your entity you fetch it with a sort descriptor.

Step 1

Add the sort attribute, in this case I named it "displayOrder"

Step 2

Append or insert the new item to your list.

 let insertAt = 0
 managedContextVar.insert(mngdObjectVar, atIndex: insertAt )

Step 3

Resort everything in that entity and update their sort value.

func updateDisplayOrder() {
    for i in 0..<taskList_Cntxt.count {
        let task = taskList_Cntxt[i]
        task.setValue( i, forKey: "displayOrder" )
    }
}

Then save!

Step 4

Now, next time you do a fetch request, make sure you add in the sort.

   let sortDescriptor = NSSortDescriptor(key: "displayOrder", ascending: true )

That's it!

That should handle your sort fairly well. To be honest, sometimes this code does insert my new items second from the top instead of the top, but 95% of the time this code works great. Regardless, you get the idea and can tweak and improve it.

Good luck.

来源：https://stackoverflow.com/questions/2707905/retrieve-core-data-entities-in-order-of-insertion