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Update: This is even trickier! Don’t forget thread savety in mergeContextChanges, or it will crash absolutely unpredictable!
This was a tricky one.
If you edit a context in another thread and save it, your main context does not get the change notifications. You can either refetch the data (into a fetchedResultsController for example) OR you use mergeChangesFromContextDidSaveNotification.
But [...]

ObjC’s concept of categories has some problems when you use it too much – name conflicts!
Best practice is to prefix your own private categories (as there are no name spaces).
And you should really do that – some iPhone Apps are rejected currently for using categories that override default code.
Set the environment variable OBJC_PRINT_REPLACED_METHODS to YES to [...]

Some notes I thought they are important, delivered to you from Hamburg

design with ui in mind
normalisation <-> denormalisation
willful introduction of repetition for performance
dont do inheritance
put blogs where they belong
prefetching to get attributes more instantly
setrelationshipkeypathsforprefetching, pass nsarray with “Songs” …
batching: only deal with whats needed
[fetch setFetchBatchSize:10]
Nsarray still responds to count!!!
Array faulting those objects in [...]