Tuesday, November 13, 2012

When Magical Record stops being magical

Some time ago i discovered Magical Record. It offers some fantastic categories to help you setup your Core Data stack with a single line of code. But it's much more than a Core Data boilerplate thingy. It also offers a range of Active Record pattern like shorthand methods. For example you can find all the instances of a particular type of object without having to write your own NSPredicate and NSFetchRequest using MR_findAll.

Somewhat recently they moved to version 2.0, along with lots of others features they added support for Nested object contexts. If you haven't read about nested/ parent-child managed object context your missing out, especially if you've ever tried to use Core Data with multiple threads.

So when did it stop becoming magical?

Usually i use Magical Record to setup my core data stack, and take advantage of its singleton default context, for those times when i'm too lazy to pass it around like a good engineer. I've also been known to dabble a little with it's Active Record implementation.

However with 2.0, the developer(s) have chosen to change the internal structure slightly. A "Root" context is created at the top of the tree, this has a child context that is returned when you call the MR_defaultContext method. The root context controls the writing of data to disk. This means that if you attempt to save the MR_defaultContext your changes are not persisted to disk, but instead pushed up to the root context.

Well that awesome, right ...

Well it is, if it was obvious. Instead I was left no errors, an empty sqlite db, and no data. Personally I really think they should have just made the MR_defaultContext the root context, and allowed users to nest contexts themselves if they wish. In case your interested you can reach the 'Root' context by calling MR_rootSavingContext.

Anyways after digging around the source code, and a little RTFM I discovered that in order to actually persist your data to disk you have to call MR_saveNestedContexts. This method will recursively call up the tree and save each context. Neat, but not obvious, especially when the API convention is to simply call the native save:

So really this post exists to say, if your a fan of Magical Record and your pulling your hair wondering why save: has stopped working the answer is use MR_saveNestedContexts.


Anonymous said...

Thank you! I've been messing around with this for hours.

Calvin G. said...

"the answer is use MR_saveNestedContexts"

In case someone's stumbling upon this, it's worth noting MR_saveNestedContexts is being deprecated for Magical Record 3.0

(see https://github.com/magicalpanda/MagicalRecord/blob/develop/Docs/Saving.md)