Why might changes be populated from one NSManagedObjectContext to another without an explicit merge?

Question

I'm working on an object import feature that utilizes multiple threads/NSManagedObjectContexts, using http://www.mac-developer-network.com/columns/coredata/may2009/ as my guide (note that I am developing for iPhone).

For some reason, when I save one of my contexts the other is immediately updated with the changes, even though I have commented out my calls to mergeChangesFromContextDidSaveNotification. Are there any reasons the contexts might be merging into one another without an explicit call?

Here a log of what's going on:

// 1.) Main context is saved with "Peter Gabriel"
// 2.) Test context is created, begins with same contents as main context
// 3.) Main context is inserted with "Spoon"
// 4.) Test context is inserted with "Phoenix"

// Contents at this point:

CoreTest[4341:903] Artists in main context: (
    "Peter Gabriel",
    "Spoon"
)
CoreTest[4341:903] Artists in test context: (
    "Peter Gabriel",
    "Phoenix"
)

// 5.) testContext is saved

// New contents of contexts:

CoreTest[4341:903] Artists in main context: (
    "Peter Gabriel",
    "Phoenix",
    "Spoon"
)
CoreTest[4341:903] Artists in test context: (
    "Peter Gabriel",
    "Phoenix"
)

As you can see, the test context is saved midway through, and the main context suddenly has the new objects from the test context, even though I haven't performed the whole NSManagedObjectContextDidSaveNotification/mergeChangesFromContext combo.

My understanding is that no changes will ever be merged unless done so explicitly... does anyone know what's going on here?

Answer 1

My understanding is that no changes will ever be merged unless done so explicitly...

That's not correct. I guess after your 5) you refetched Artists in the main context, right? Any fetch always goes down to the disk, accessing the file. If that's after the save from a different context, that'll fetch new entries you just created. If there're conflicts, those will be dealt with according to your mergePolicy, see this Apple doc.