Android Realm.io迁移：回退到.deleteRealmIfMigrationNeeded（）

Question

When migrating an Android Realm.io instance to a newer schema I provide migration steps in my Migration implementation:

  RealmConfiguration config = new RealmConfiguration
            .Builder(this)
            .schemaVersion(SCHEMA_VERSION)
            .migration(new Migration())
            .build();

What I want to do in the actual migration code I want to fallback to deleteRealmIfMigrationNeeded for older schema versions.

Is there any way to do that? I tried to do it with deleteAll() but that doesn't seem to work as some people updating from an older version of the app are getting Realm validation errors.

public class Migration implements RealmMigration {
    @Override
    public void migrate(DynamicRealm realm, long oldVersion, long newVersion) {

        if (oldVersion < 105) {
            realm.deleteAll();
            return;
        }

        //handle newer schema versions


    }
}

Answer 1

You can try to open a Realm in dynamic mode and ask for it's version. DynamicRealms will not trigger migrations:

RealmConfiguration config = new RealmConfiguration
            .Builder(this)
            .schemaVersion(SCHEMA_VERSION)
            .migration(new Migration())
            .build();

// Use DynamicRealm to find version and delete it if it is too old
DynamicRealm dRealm = DynamicRealm.getInstance(config);
boolean delete = dRealm.getVersion() < 42;
dRealm.close();
if (delete) {
  Realm.deleteRealm(config);
}

Realm realm = Realm.getInstance(config);

Answer 2

To clear the classes within the migration, you can remove all the classes that are within the Realm, and then restore classes using the same methodology that Realm uses:

package io.realm;

public class Migration implements RealmMigration {
    @Override
    public void migrate(DynamicRealm realm, long oldVersion, long newVersion) {
        RealmConfiguration configuration = realm.getConfiguration();
        if(oldVersion < 105) {
            RealmSchema schema = realm.getSchema();
            Set<RealmObjectSchema> classSchemas = schema.getAll();
            for(RealmObjectSchema classSchema : classSchemas) {
                schema.remove(classSchema.getClassName());
            }
        }
        Set<Class<? extends RealmModel>> currentModels = configuration.getRealmObjectClasses();
        Set<Class<? extends RealmModel>> nonExistentModels = new HashSet<>();
        for(Class<?> clazz : currentModels) {
            if(!schema.contains(clazz.getCanonicalName()) {
                nonExistentModels.put(clazz);
            } else {
                // TODO: migrate current classes that do exist
            }
        }

        for(Class<?> nonExistentClazz : nonExistentModels) {
            // partly taken from Realm 1.1.1's Realm.java : initializeRealm(Realm) method
            RealmProxyMediator mediator = configuration.getSchemaMediator(); // package internal
            mediator.createTable(modelClass, realm.sharedGroupManager.getTransaction()); // package internal
        }
    }
}

Although to be fair, this uses package-internal magic and Christian Melchior's answer is simpler, and doesn't rely on such magical things.

Also, I wrote it directly here, so I haven't actually ran it.

Answer 3

How about something to the effect of:

    final RealmConfiguration.Builder builder=new RealmConfiguration.Builder(this);
    builder.schemaVersion(SCHEMA_VERSION);
    if (SCHEMA_VERSION < 105) {
       builder.deleteRealmIfMigrationNeeded();
    } else {
       builder.migration(new Migration());
    }
    final RealmConfiguration config = builder.build();