Nice...

async function onNewMessage(newMessage) {
  const [
    seenTimestamp,
    previousMaxMessageTimestamp
  ] = await Promise.all([
    r.table('RoomUser').get(['r31', 'liron-shapira'])('seenTimestamp'),
    r.table('Message').filter({roomId: 'r31'}).max('timestamp')
  ])

  if (previousMaxMessageTimestamp < seenTimestamp && seenTimestamp < newMessage.timestamp) {
    // r31's status is flipping from "read" to "unread" as a result of inserting newMessage,
    // so we know that if we were to recalculate numUnreadRooms, it would end up incrementing
    // the previous value by 1
    await r.table('User').get('liron-shapira').update({
      numUnreadRooms: r.row('numUnreadRooms').add(1)
    })
  }
}