rethinkdb create a read only copy on a tagged server

r.db("rethinkdb").table("table_config")
  .hasFields('db', 'name')
  .concatMap(doc => [{
    db: doc('db'),
    table: doc('name'),
    shards: doc('shards').count(),
    replicas: doc('shards').nth(0)('replicas').count(),
  }])
  // this part is dumb - assuming you have 4 main servers, 
  // and that ONLY tables with 5 total replicas have 5 replicas
  .filter(doc => doc('replicas').lt(5))
  .forEach(doc => r.db(doc('db')).reconfigure({
    shards: doc('shards'),
    replicas: {'main': doc('replicas'), 'ro': 1},
    primary_replica_tag: 'main'
  }))