Ruby ActiveRecord: How to connect two rooms with an exit - ruby

Question

I am making a small text-adventure, and I want to use ActiveRecord as Object Relational Mapping.
What I'm having trouble with, is understanding how to connect two rooms together using an exit.
The following facts are a given:
A room can have multiple exits
An exit can be in different directions (it has a 'direction' field). Also, it might have other parameters such as 'locked', etc. that I want to add later.
An exit connects up two rooms.
However, now I am stuck:
What I have so far
class Room < ActiveRecord::Base
has_many :exits
has_many :neighbours, through: :exits
end
class Exit < ActiveRecord::Base
belongs_to :room, dependent: :destroy
belongs_to :room_dest, foreign_key: "room_dest_id", class_name: "Room", dependent: :destroy
end
but this is incomplete. room.neighbours, for example, is not working at all.
What mostly baffles me is how to make exits work two-ways: If I add an exit on one room, it won't be in the room.exits list in the other room.
What works is: (given an exit connecting room1 and room2)
room1.first.exits.first.room_dest (this is room2)
But room2.exits is empty, and room1.neighbours shows a list containing only itself.
How is this done properly?

Answer 1

To get room.neighbours working, I believe you would first need to change
belongs_to :room_dest, foreign_key: "room_dest_id", class_name: "Room", dependent: :destroy
to
belongs_to :neighbour, foreign_key: "room_dest_id", class_name: "Room"
Note that I removed the dependent: option because you probably don't want to destroy Rooms when Exits are deleted. You want dependent: :destroy on the has_many relationship with exits.
Now we've really solidified the one-way binding of Exit. But if you think about it, isn't "Exit" just one-way by definition? While this seems limiting at first, you can utilize this to define "entrances" of the Room. That is your connection from the neighbour to the original Room. Something like:
has_many :entrances, class_name: "Exit", foreign_key: "room_dest_id"
Or you could define a method that queries Exits and checks if either room_id or room_dest_id is the Room ID. In this case I would rename "Exit" class to be something more generic. Unfortunately I can't think of any built-in AR association that would do this multi-key association for you. It wouldn't really work right because things like association.build/create would not know which of the keys to set. But it's a relatively simple method or set of methods that could still return a scope which you could operator on:
has_many :connections, dependent: :destroy # For ease of creating connection, not as useful for querying them
def exits
Connection.where(["room_id = :id OR dest_room_id = :id", id: self.id])
end
def neighbours
exits.map do |conn|
conn.room_id == self.id ? conn.dest : conn.source
end
end
You could make the query more complex if you actually want some of the Connections to be one-way. Or write other methods that build on this since it is not executed immediately. You could still chain .first(), another .where(), etc on it.