MigrationAfter.java

@Reusable
public class Migration implements RealmMigration
{
    private Map<Integer, Provider<VersionMigration>> versionMigrations;

    @Inject
    Migration (Map<Integer, Provider<VersionMigration>> versionMigrations)
    {
        this.versionMigrations = versionMigrations;
    }

    @Override
    public void migrate (final DynamicRealm realm, long oldVersion, long newVersion)
    {
        for ( int i = ( int ) oldVersion; i < newVersion; i++ )
        {
            final Provider<VersionMigration> provider = versionMigrations.get( i );
            if ( provider != null )
            {
                VersionMigration versionMigration = provider.get();
                versionMigration.migrate( realm, i );
            }
        }
    }

}

MigrationsModule.java

@Module
public class MigrationsModule
{
    @Provides
    @IntoMap
    @IntKey( 1 )
    static VersionMigration provideVersion1Migration ()
    {
        return new Version1Migration();
    }

    @Provides
    @IntoMap
    @IntKey( 2 )
    static VersionMigration provideVersion2Migration ()
    {
        return new Version2Migration();
    }
}

Version1Migration.java

class Version1Migration implements VersionMigration
{
    private static final String RECIPE = "Recipe";

    /************************************************
     // Version 2

     class Recipe
     Integer numberOfStars //added
     ************************************************/
    @Override
    public void migrate (DynamicRealm realm, long oldVersion)
    {
        if ( oldVersion == 1 )
        {
            RealmObjectSchema recipeSchema = getObjectSchema( realm );
            recipeSchema.addField( RecipeFields.NUMBER_OF_STARS, Integer.class )
                        .transform( new RealmObjectSchema.Function()
                        {
                            @Override
                            public void apply (DynamicRealmObject obj)
                            {
                                obj.setInt( RecipeFields.NUMBER_OF_STARS, 5 );
                            }
                        } );

            Timber.d( "migration complete" );
        }
    }

    RealmObjectSchema getObjectSchema (DynamicRealm realm)
    {
        RealmSchema schema = realm.getSchema();
        return schema.get( RECIPE );
    }
}

Version1MigrationTest.java

public class Version1MigrationTest
{
    @Rule
    public MockitoRule rule = MockitoJUnit.rule();

    @Mock
    DynamicRealm dynamicRealm;

    @Mock
    RealmObjectSchema realmObjectSchema;

    private Version1Migration migration;

    @Before
    public void setUp ()
    {
        migration = new Version1Migration()
        {
            @Override
            RealmObjectSchema getObjectSchema (DynamicRealm realm)
            {
                return realmObjectSchema;
            }
        };
    }

    @Test
    public void shouldNotBeNull ()
    {
        assertNotNull( migration );
    }

    @Test
    public void migrate_shouldDoNothing_whenNotProperOldVersion ()
    {
        migration.migrate( dynamicRealm, 2 );

        verifyZeroInteractions( dynamicRealm );
    }

    @Test
    public void migrate_shouldAddField ()
    {
        when( realmObjectSchema.addField( anyString(), eq( Integer.class ) ) ).thenReturn( realmObjectSchema );

        migration.migrate( dynamicRealm, 1 );

        verify( realmObjectSchema ).addField( RecipeFields.NUMBER_OF_STARS, Integer.class );
        verify( realmObjectSchema ).transform( any( RealmObjectSchema.Function.class ) );
    }

}

VersionMigration.java

interface VersionMigration
{
    void migrate (final DynamicRealm realm, long oldVersion);
}