Cannot get the latest CoreData DB file in Swift

Question

I have built a app using CoreData for persisting contents. Everything works fine. Now I would like to export the SQLite DB file of my stored data.

I checked the DB file's path by downloading container from my device. It seems to be "~\Library\Application Support\MainData.sqlite".

Screenshot of filepath:

I called shareDatabase() to share DB file using AirDrop to my Mac, and it works normally:

func shareDatabase() {
    try? (UIApplication.shared.delegate as! AppDelegate).persistentContainer.viewContext.save()
    let fileName = "MainData.sqlite"
    let filePath = URL(fileURLWithPath: NSHomeDirectory()).appendingPathComponent("Library").appendingPathComponent("Application Support").appendingPathComponent(fileName)

    let activityVC = UIActivityViewController(activityItems: [filePath], applicationActivities: nil)
    present(activityVC, animated: true, completion: nil)
}

Strangely, the transferred MainData.sqlite is not the latest one, but the one with modified date of yesterday. I also used DB Browser opened the file. It does not contain the latest data.

Screenshot: info of AirDrop-shared file:

However, the MainData.sqlite from downloaded container is latest updated. The latest data are in this file, checking with DB Browser.

Screenshot: info of file in container:

Please help point out what is wrong with my codes.

---

Added contents:

Thank @user2782993 for referring the Apple answer on the topic: https://developer.apple.com/library/content/qa/qa1809/_index.html

I also read about this technical Q&A from apple library. But it is not very clear how to implement the mode changing option in XCode generated lazy var persistentContainer.

Also, this thread mentioned,

For sanity sake you should ensure you only have one connection to the persistent store when you do this, i.e. only one persistent store instance in a single persistent store coordinator.

I'm not sure how to close the existing connection. Any ideas? (better in Swift)

Answer 1

Try to use this:

https://stackoverflow.com/a/18870738/2782993

or this:

https://stackoverflow.com/a/20252663/2782993

Check this out:

http://pinkstone.co.uk/how-to-remove-wal-files-in-core-data/

Excerpt:

"Since iOS 7 and OS X 10.9 the default journalling mode in SQLite Stores is WAL. In addition to the main store file you’ll find a WAL file with the same (or larger) size as the store file, and a less important SHM file.

Prior to this implementation it was easy to save the context, extract the store file and ship it with an app as a pre-made data store. That’s no longer possible, because by default all data changes are written to the WAL file and do not sync with the main store file.

This is not a problem if you’re not shipping a pre-made store file with your app, but if you do, then this “improvement” has just ruined your way of delivering prewritten data stores.

Lucky for us we can switch this entire WAL business off by passing an option when creating our NSPersistentStoreCoordinator."

Here is the Apple answer on the topic:

https://developer.apple.com/library/content/qa/qa1809/_index.html

Please notice, synching the WAL data is called a checkpoint operation. The important parts are in bold

Excerpt:

"A: The failure occurs because the default journaling mode for Core Data SQLite stores was changed to Write-Ahead Logging (WAL) in iOS 7 and OS X Mavericks. With the WAL mode, Core Data keeps the main store file untouched and appends transactions to a -wal file in the same location. After the Core Data context is saved, the -wal file is not deleted, and the data in that file is not merged to the store file either. Therefore, simply making copies of the store file will likely cause data loss and inconsistency.

...

To safely back up and restore a Core Data SQLite store, you can do the following:

Use the following method of NSPersistentStoreCoordinator class, rather than file system APIs, to back up and restore the Core Data store:

- (NSPersistentStore *)migratePersistentStore:(NSPersistentStore *)store toURL:(NSURL *)URL options:(NSDictionary *)options withType:(NSString *)storeType error:(NSError **)error

Note that this is the option we recommend.

Change to rollback journaling mode when adding the store to a persistent store coordinator if you have to copy the store file. Listing 1 is the code showing how to do this:

Listing 1 Use rollback journaling mode when adding a persistent store

NSDictionary *options = @{NSSQLitePragmasOption:@{@"journal_mode":@"DELETE"}};
if (! [persistentStoreCoordinator addPersistentStoreWithType:NSSQLiteStoreType
                                        configuration:nil
                                        URL:storeURL
                                        options:options
                                        error:&error]) {
    // error handling.
}

For a store that was loaded with the WAL mode, if both the main store file and the corresponding -wal file exist, using rollback journaling mode to add the store to a persistent store coordinator will force Core Data to perform a checkpoint operation, which merges the data in the -wal file to the store file. This is actually the Core Data way to perform a checkpoint operation. On the other hand, if the -wal file is not present, using this approach to add the store won't cause any exceptions, but the transactions recorded in the missing -wal file will be lost. Bundle the main store file and the -wal file into a document package and manipulate them as a single item. For more information about the default journaling mode change, please see WWDC 2013 session 207 What's New in Core Data and iCloud.

NOTE: In iOS 6.x and Mountain Lion, the default is rollback journaling mode, in which Core Data creates a -journal file to temporarily store transactions, updates the main store file in place and deletes the -journal file after saving the context. The store file therefore contains the up-to-date database."