From: Todd Benson <caduceass@...>
Date: 2008-10-06T08:46:47+09:00
Subject: Re: Object Persistence for a MUD

On Sat, Oct 4, 2008 at 8:46 PM, Brent Dillingham
<brentdillingham@gmail.com> wrote:
> Hi all,
>
> For fun, I'm writing a MUD with Ruby. I thought I'd query the hive
> mind here and ask some opinions on what library I might use for object
> persistence -- that is, storing objects like players and rooms
> somewhere.
>
> I could do something really simple like store everything in YAML, I
> could try DRb, or I could go all out and use an ORM like DataMapper...
> DataMapper is the first route I took, but I soon realized that I'll
> have to be really careful when using a lot of the ORM niceties while
> still ensuring that a database row only every represents one object in
> memory. i.e.:
>
> # assuming Player has_one :room ...
>
> player = Player.get(1)      # get Player with ID=1
> player.room                 # the player's current Room association
>
> ... player logs out, logs back in later ...
>
> player.room
>
> Oh noes! The ORM fetches the same room from the DB, but creates a
> totally new object to represent it. Two players could be in the same
> room ID, but actually reference different objects, and thus couldn't
> see or interact with one another. Bad.
>
> This problem is hackable by carefully keeping a cache of loaded
> objects -- but might end up negating the value of using an ORM like
> DataMapper. So, any suggestions out there of what I might do instead?

There are probably a hundred different ways to approach this.  I've
never programmed a MUD, but after playing around with the Sequel gem
and Postgresql, I had a simple map with locations, players, and
objects.  I updated the DB on the fly, but I'm sure there are other
paradigms.

Very, very simple example...

sql...

create database mud;

create table map (
  x int not null,
  y int not null,
  synopsis varchar not null,
  description varchar
  primary key (x, y)
);

create table people (
  name varchar not null primary key,
  xcurrent int not null,
  ycurrent int not null,
  foreign key (x, y) references map (x, y)
);

insert into map values (0, 0, 'start');
insert into map values (1, 0, 'hallway');
insert into people values ('Loki', 1, 0);
insert into people values ('Thor', 0, 0);
insert into people values ('Freya', 1, 0);

...ruby...

require 'rubygems'
require 'sequel'
Area = Struct.new(:description, :synopsis, :x, :y)
DB = Sequel.connect("postgres://user_name:password@localhost:5432/mud")

#print everyone in the hallway...

DB[:map].join(:people, :xcurrent => :x, :ycurrent => :y).where (:x =>
1, :y => 0).all.each {|row| p row[:name]}

#=> Loki
#=> Freya


Todd