"On duplicate key update" for core-data

I would like to know if开发者_开发问答 there is some kind of similar functionality or way to preform an "on duplicate key update" function with core-data as there is with MySQL.

What I want to do is to save an object to the database every time a user presses a button. But if the button is already pressed I want to update the row with some new values instead of adding a new row.

The only way I currently know how to do this is to read the rows from the DB, see if the row exists and then update it.. otherwise add a new row. This seems kind of bad to do this way, am I wrong?

The easiest answer to this is to run a query against the Core Data context and get the object back if it exists. This is the most efficient and least error prone solution to the problem.

You do not need to create a separate NSManagedObjectContext and attempt to deal with merge policies, that is a very inefficient and dangerous way to try and resolve such a simple issue.

Core Data handles a lot of caching for you in the background. If you are attempting to retrieve an object that you just created there is a very high probability that it is still sitting in the cache so the response to your query will be nearly instantaneous.

Note

I just went back to both of those sample projects again to file a bug against them and noticed that they have been updated and finally removed the suggestion of creating a new context. They are now using the NSUndoManager where appropriate and a single context.

EDIT

Remember that the core data framework manages persistence of your object graph. It is not an interface to a sqlite database.

Worry about your object life cycle. When do instances get created? When are they destroyed? What makes your instances unique? Using Books as an example entity, a book has an ISBN which is a unique way of identifying a title, yet many copies of each title can exist. You have two choices in your Entity model, you can create separate instances for each copy of the title or have one instance with a count attribute.

The sample projects CoreDataBooks and iPhoneCoreDataRecipes use NSUndoManager to track state changes between views.