Versioning and Migration - Core Data: Data Storage and Management for iOS, OS X, and iCloud - page 55

In this code segment, we first retrieve the metadata from the source URL. If
that metadata is not nil , we ask the final model whether the metadata is
compatible with it. If it is, we are happy and done. We then set the error
pointer to nil and return YES . If it isn't compatible, we need to try to figure out
the mapping model and potentially the interim data model to migrate to.
Finding All the Managed Object Models
To proceed to the next step in the migration, we need to find all the managed
object models in the bundle and loop through them. The goal at this point is
to get all the models and figure out which one we can migrate to. Since these
models will probably be in their own bundles, we have to first look for the
bundles and then look inside each of them.
//Find the source model
NSManagedObjectModel *sourceModel = [NSManagedObjectModel
mergedModelFromBundles:nil
forStoreMetadata:sourceMetadata];
NSAssert(sourceModel != nil, ([NSString stringWithFormat:
@ "Failed to find source model\n%@" ,
sourceMetadata]));
//Find all of the mom and momd files in the Resources directory
NSMutableArray *modelPaths = [NSMutableArray array];
NSArray *momdArray = [[NSBundle mainBundle] pathsForResourcesOfType:@ "momd"
inDirectory:nil];
for (NSString *momdPath in momdArray) {
NSString *resourceSubpath = [momdPath lastPathComponent];
NSArray *array = [[NSBundle mainBundle]
pathsForResourcesOfType:@ "mom"
inDirectory:resourceSubpath];
[modelPaths addObjectsFromArray:array];
}
NSArray* otherModels = [[NSBundle mainBundle] pathsForResourcesOfType:@ "mom"
inDirectory:nil];
[modelPaths addObjectsFromArray:otherModels];
if (!modelPaths || ![modelPaths count]) {
//Throw an error if there are no models
NSMutableDictionary *dict = [NSMutableDictionary dictionary];
[dict setValue:@ "No models found in bundle"
forKey:NSLocalizedDescriptionKey];
//Populate the error
*error = [NSError errorWithDomain:@ "Zarra" code:8001 userInfo:dict];
return NO;
}
In this code block, we first grab all the resource paths from the mainBundle that
are of type momd . This gives us a list of all the model bundles. We then loop
through the list and look for mom resources inside each and add them to an