Context is important. Also future-proofing.
If your app was originally written against v1 CoreData (Tiger), you
need to update the app to be wise enough to check the store's metadata
and run away, run away from any store containing unexpected values (ie
version hash information). v1 CoreData always assumes that it can open
a store it was told to open until later evidence proves otherwise;
later versions of CoreData realized that was perhaps an unwise
decision, and added the current versioning and migration
infrastructure. Due to binary compatibility issues, applications
compiled on Tiger will continue to exhibit the old behavior. There's a
general discussion of migration under 10.4 at http://developer.apple.com/documentation/Cocoa/Conceptual/CoreData/Articles/cdZ104Versioning.html#//apple_ref/doc/uid/TP40002989
Later versions of CoreData always check the store metadata for version
hash information, which is affected by things that could change the
schema (adding and removing entities and/or persistent properties,
adding or removing validation rules, etc), but not by things that
don't change the schema (adding or removing transient properties,
adding or removing default values). For more details, see the Core
Data Model Versioning and Data Migration Programming Guide, and for a
concrete discussion of what affects version hashes, the section on
versioning (http://developer.apple.com/documentation/Cocoa/Conceptual/CoreDataVersioning/Articles/vmVersioning.html#//apple_ref/doc/uid/TP40004714-SW1
).
An application controlled technique would be to add an application
specific piece of metadata (a version # perhaps) to your stores, and
have your various application versions categorically refuse to open a
store whose app version is >= the version it knows how to read. This
can allow for a better user experience than the default
NSPersistentStoreIncompatibleVersionHashError returned by
addPersistentStore, which is a bit vague about which newer version of
the application you need to buy to open a given file.
Cheers,
+Melissa
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