Store Metadata

You can associate metadata with a persistent store. The template code for the Spotlight importer checks to see if the store file is what should be indexed. If so, it reads the relevant store metadata and passes appropriate values to the Spotlight indexer.

The metadata is a dictionary of key-value pairs. Core Data automatically sets values for NSStoreType and NSStoreUUID. To set custom keys and values, you make a mutable copy of the existing dictionary, add your own key-value pairs that will be read by the importer, and then set the updated dictionary as the metadata for the store.

You should be careful what information you put into metadata for two reasons:

• Spotlight imposes a limit to the size of metadata.

• Replicating an entire document in metadata is probably not useful.

Recall that it is possible to create a URL to identify a particular object in a store (using URIRepresentation)—the URL may be useful to include as metadata. Large metadata is best stored separately from the database file. For example, cached images for QuickLooks are much cheaper to store in a dedicated file on disk that can be memory mapped in, than stored in the database's metadata.

Setting Metadata

You set the metadata for a store using the NSPersistentStore method setMetadata:forPersistentStoreWithURL:error:.

NSError *error = nil;

NSURL *storeURL = <#URL for persistent store#>;

NSDictionary *metadata =

    [NSPersistentStore metadataForPersistentStoreWithURL:storeURL error:&error];

if (metadata == nil) {

    /* Deal with the error. */

}

else {

    NSMutableDictionary *newMetadata = [metadata mutableCopy];

    [newMetadata setObject:@[@"MyKeyWord"] forKey:(__bridge id<NSCopying>)kMDItemKeywords];

    // Set additional key-value pairs as appropriate.

    [NSPersistentStore setMetadata:newMetadata

                       forPersistentStoreWithURL:storeURL

                       error:&error];

}

Note that setting the metadata for a store does not change the information on disk until the store is actually saved.

Reading Metadata

NSPersistentStoreCoordinator provides a class method, metadataForPersistentStoreWithURL:error:, that allows you to retrieve metadata from a store without the overhead of creating a Core Data stack. You can also use the NSPersistentStore method metadataForPersistentStoreWithURL:error:. You use these methods in the importer to extract the data to pass to the indexer.