map into a rethink embedded doc to create index

rethinkmapindex.java

r.db(Rethink.DB).table(Rethink.SOCIAL_USER_TABLE)
                    .indexCreate(Rethink.USER_APPID_PROVIDER_PROVIDERID_IDX,
                            user -> user.g("credentials").map(
                                    c -> r.array(user.g("appId"), c.g("provider"), c.g("providerId"))
                            ))
                    .optArg("multi", true).run(connection);