Allow configuring a Realm file to be deleted when a migration is required

[pronebird]

I have a cache storage which is ok to be wiped on schema changes. I don't want to migrate it since it's transitory anyway. Is there any way to nuke my realm when schema inconsistency found? This would also help a lot during development when schema changes occur pretty often.

I found a workaround but that requires a version bump, which I can probably simulate by using bundle version. What would be the official way? :)

#import <Realm/RLMObjectSchema.h>

[RLMRealm setSchemaVersion:1 forRealmAtPath:realmPath withMigrationBlock:^(RLMMigration *migration, NSUInteger oldSchemaVersion) {
    for(RLMObjectSchema *objectSchema in migration.oldSchema.objectSchema) {
        [migration enumerateObjects:objectSchema.className block:^(RLMObject *oldObject, RLMObject *newObject) {
            [migration deleteObject:newObject];
        }];
    }
}];

RLMRealm  *cleanAndTidy = [RLMRealm realmWithPath:realmPath];

The better option I found is to nuke realm when things go south:

#import <Realm/RLMConstants.h>

static void RLMCleanupFiles(NSString *path) {
    // delete files starting with xxx.realm* name
}

@try {
    cacheRealm = [RLMRealm realmWithPath:realmPath];
}
@catch(NSException *exception) {
    NSLog(@"Caught exception while opening Realm: %@", exception);

    if([exception.name isEqualToString:RLMExceptionName]) {
        RLMCleanupFiles(realmPath);
        cacheRealm = [RLMRealm realmWithPath:realmPath];
    }
    else {
        @throw; // rethrow
    }
}

[segiddins]

@pronebird we're going to be introducing the concept of an RLMConfiguration that will likely give you the ability to specify a "delete if migration needed" option. Until then, I can't really think of a great way to do this in Swift.

[alazier]

I'm not sure I like the idea of specifying this in RLMConfiguration. I think in this case it might be nice to add a new method -clearRealm to the RLMMigration object.

[jpsim]

I think in this case it might be nice to add a new method -clearRealm to the RLMMigration object.

That would require increasing the schema version and defining a migration block, which I don't think would serve the needs prompting users to ask for this.

As much as I'd like to discourage the behavior of deleting realms on schema mismatch, the truth is that users are already finding hacky and dangerous ways to do this (try/catch block around first RLMRealm access). Maybe we should instead publicly expose something like +[RLMRealm requiresMigrationAtPath:encryptionKey:error:].

[lordkev]

Just to chime in, as we're running into this too. Currently our use case is for alpha/beta versions of an app in development. We'd prefer not to bother with migrations every time we change the schema in a non-production app.

Additionally, our use of Realm is as a persistent local cache that can be re-created if necessary. I understand discouraging the behavior of deleting realms on schema mismatch, but it's also definitely useful to have deleting the realm as an option of last resort if a migration goes south for some reason. We do a similar thing with Core Data in another app, and although it's considered the "nuclear" option, it's better than crashing at every startup.

[jpsim]

#1723 is now pending review, which should enable safely deleting a realm file on schema mismatch.

[ikonst]

@jpsim #1723 was closed without merging. Now all hope is on #1584. Is it coming up?

[jpsim]

Now all hope is on #1584. Is it coming up?

That's still our preferred direction, but no one on our team is actively working on it at the moment. However, since this functionality can be built entirely at the Objective-C level, it's actually a task anyone from the community could contribute to Realm, if you're looking to get involved.

We're currently quite involved with fine-grain notifications, KVO, cascading deletes, thread handover and preparing an official release with functionality that recently landed in master.

[ikonst]

I just might.

[segiddins]

This will be trivial to implement via RLMRealmConfiguration once #2252 is merged

[shfishburn]

How do we implement this in Swift 2.0?

[bdash]

Using Realm.Configuration.deleteRealmIfMigrationNeeded.

[shfishburn]

Read that before. If I may ask, where do we inject that line?

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Date: July 29, 2016 at 14:08:19
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Subject: Re: [realm/realm-cocoa] Allow configuring a Realm file to be
deleted when a migration is required (#1692)

Using Realm.Configuration.deleteRealmIfMigrationNeeded

https://realm.io/docs/swift/latest/api/Classes/Realm/Configuration.html#/s:vVC10RealmSwift5Realm13Configuration28deleteRealmIfMigrationNeededSb
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[jpsim]

If I may ask, where do we inject that line?

@shfishburn you pass a Realm.Configuration value when initializing a Realm: Realm.init(configuration:). So wherever you're opening a Realm for the first time in your app, you should make sure that the configuration value used has the deleteRealmIfMigrationNeeded value set to what you want it.

[NikKovIos]

Here is how to use it. Thumbs up if did help.

didFinishLaunchingWithOptions {
     let configuration = Realm.Configuration(deleteRealmIfMigrationNeeded: true)
     Realm.Configuration.defaultConfiguration = configuration
     do { _ = try Realm() } catch {}
}

    RLMRealmConfiguration *config = [RLMRealmConfiguration defaultConfiguration];
    config.deleteRealmIfMigrationNeeded = YES;
    config.schemaVersion = 2; 
    [RLMRealmConfiguration setDefaultConfiguration:config];
    [RLMRealm defaultRealm];
    NSLog(@">>> Realm path:  %@", [config fileURL]);