The application

LiveRoom is all about posting topics and messages. When users sign in, they can start discussions by creating new topics. The newly created topics will be shown in the list of most recent discussions, namely the latest discussions list. If a user clicks on a topic, he/she will enter the discussion and get a list of messages related to the topic. If he/she has signed in, he/she will be able to post a message related to the topic that he/she selected, or comment on an existing message. The following figure shows the data model of topic, message, and comment:

And here are the type definitions in Opa:

type Topic.t = {
  int   id,    //id, the primary key
  string title,    //title of the topic
  string author,  //the author
  int   posttime,  //post time
  int   lastupdate,  //last update time, 
  string lastposter,  //last poster
  int   reply,    //number of replies
  stringmap(Message.t) messages
}
type Message.t = { 
string author, string content, int posttime, 
  list(Comment...