Load a previous model version

Question

I am loading a NSManagedObjectModel model with the initWithContentsOfURL: constructor like this:

NSURL *modelURL = [[NSBundle mainBundle] URLForResource:@"MyDocument" withExtension:@"momd"];
NSManagedObjectModel *mom = [[NSManagedObjectModel alloc] initWithContentsOfURL:modelURL];

However this only gives me access to the latest/current version of a model. Is it posible to load previous versions with the same momd file? how?

Answer 1

Actually you can load an older version with:

- (NSManagedObjectModel *)managedObjectModelForVersion:(NSString *)version
{
        NSURL *modelURL = [[NSBundle mainBundle] URLForResource:[NSString stringWithFormat:@"AppModel.momd/AppModel %@",version] withExtension:@"mom"];
        NSManagedObjectModel *model = [[NSManagedObjectModel alloc] initWithContentsOfURL:modelURL];
        return model;
 }

Just replace AppModel with your model name.

I'm using this to get myself out of a sticky manual migration situation involving iCloud. Searched high and low and couldn't find this anywhere.

Answer 2

If you just want to load the version of the model that's compatible with a particular existing store try:

NSError *error = nil;
NSDictionary *storeMetadata = [NSPersistentStoreCoordinator metadataForPersistentStoreOfType:NSSQLiteStoreType 
                                                                                         URL:storeURL 
                                                                                       error:&error];
NSManagedObjectModel *oldManagedObjectModel = [NSManagedObjectModel mergedModelFromBundles:[NSArray arrayWithObject:[NSBundle mainBundle]] 
                                                                          forStoreMetadata:storeMetadata];

Note that if you use XCode version identifiers for your data model versions, the persistent store's current version identifiers are accessible through the NSStoreModelVersionIdentifiersKey entry in the store metadata dictionary.

As far as loading a particular arbitrary version is concerned, the mom files are typically located under the momd directory in your app's bundle, so you could enumerate them using NSFileManager. I believe to find one with a particular version identifier you would have to use NSManagedObjectModel's initWithContentsOfURL: initializer and then inspect the versionIdentifiers property, or use the isConfiguration:compatibleWithStoreMetadata: instance method to determine compatibility.

Answer 3

Made the solution offered by @Schoob into a category, because they rock.

@interface NSManagedObjectModel (version)
+ (NSManagedObjectModel *)modelFromBundle:(NSBundle *)bundle name:(NSString *)modelName version:(NSString *)version;
@end

@implementation NSManagedObjectModel (version)
+ (NSManagedObjectModel *)modelFromBundle:(NSBundle *)bundle name:(NSString *)modelName version:(NSString *)version
{
    if(!bundle)
        bundle = [NSBundle mainBundle];

    NSString *resource = [[modelName stringByAppendingPathExtension:@"momd"] stringByAppendingPathComponent:version];
    NSURL *modelURL = [bundle URLForResource:resource withExtension:@"mom"];
    NSAssert(modelURL,@"Unable to find MOM - %@",resource);
    NSManagedObjectModel *model = [[NSManagedObjectModel alloc] initWithContentsOfURL:modelURL];
    NSAssert(model,@"Unable to load MOM at URL- %@",modelURL);
    return model;
}
@end

Answer 4

Swift version. Replace file name.

import CoreData

extension NSManagedObjectModel
{
    class func model(forVersion version: Int) -> NSManagedObjectModel?
    {
        if let fileUrl = Bundle.main.url(forResource: "Model.momd/Model \(version)", withExtension: "mom")
        {
            return NSManagedObjectModel(contentsOf: fileUrl)
        }
        return .none
    }
}

Answer 5

No, it is not foreseen that this is possible. I deduce that from the NSManagedObjectModel documentation, where it says discussing the property versionIdentifiers:

This value is meant to be used as a debugging hint to help you determine the models that were combined to create a merged model.

So it does not seem you are supposed to use previous model versions for your program logic.