Core Data migration problem: “Persistent store migration failed, missing source managed object model.”

Question

The Background

• A Cocoa Non Document Core Data project with two Managed Object Models.
• Model 1 stays the same. Model 2 has changed, s

Answer 1

While attempting to upgrade an existing app's Core Data model (and migrate legacy data), I just ran across a scenario where a third-party Framework was writing data into an app's database. I was getting this error, "Can't find model for source store." Because the third party model was not loaded when I was attempting the migration, the migration was failing.

I wrote this method (below) during troubleshooting this issue. It may be useful to those who are facing these types of issues.

- (BOOL) checkModelCompatibilityOfStoreWithURL: (NSURL *) myStoreURL
                                 forModelNamed: (NSString *) myModelName
                                  withBasePath: (NSString *) myBasePath;
{
    NSError * error = nil;
    NSDictionary *storeMeta = [NSPersistentStoreCoordinator metadataForPersistentStoreOfType:nil URL:myStoreURL error:&error];
    if (!storeMeta) {
        NSLog(@"Unable to load store metadata from URL: %@; Error = %@", myStoreURL, error);
        return NO;
    }

    NSString * modelPath = [myBasePath stringByAppendingPathComponent: myModelName];
    NSFileManager *fileManager = [NSFileManager defaultManager];
    if (![fileManager fileExistsAtPath: modelPath]) {
        // uh oh
        NSLog(@"Can't find model.");
        return NO;
    }

    NSURL * modelURL = [NSURL fileURLWithPath: modelPath];
    NSManagedObjectModel * model = [[[NSManagedObjectModel alloc] initWithContentsOfURL: modelURL] autorelease];
    BOOL result = [model isConfiguration: nil compatibleWithStoreMetadata: storeMeta];

    NSLog(@"Tested model, %@, is%@ compatible with Database", modelPath, result ? @"" : @" ~not~");

    return result;
}

This code snippet will obtain a store's metadata.

NSError *error = nil;
NSURL *storeUrl = [NSURL fileURLWithPath:storePath];
NSDictionary *storeMeta = [NSPersistentStoreCoordinator metadataForPersistentStoreOfType:nil URL:storeUrl error:&error];
NSLog(@"%@", [storeMeta objectForKey: @"NSStoreModelVersionHashes"]);

The VersionInfo.plist (stored in the compiled app's bundle) contains the hashes that are associated with the various entities in your models (base64 encoding). Similarly, a BLOB column in the datastore (Z_METADATA.Z_PLIST) contains a binary-encoded property list that has the hashes (also base64 encoded) for each entity that is associated with the data.

The -entitiesByName method on NSManagedObjectModel is useful for dumping the entities and hashes that exist within a specific model.