Referencing foreign keys with DataMapper

I've been looking through this:

http://datamapper.org/docs/find

But haven't been able to gleam what I'm looking for, though I know it's quite simple.

I have two tables, scans and stations, with the relevant fields:

STATIONS - id (primary key), name
SCANS    - id (primary key), item_id, in_station, out_station

Where in_station and out_station are foreign keys to the id field in the stations table.

I have a Scan object

class Scan
    include DataMapper::Resource

    property :id,                       Integer, :key => true
    property :item_id,                  Integer
    property :in_station,               Integer
    property :out_station,              Integer
end

So right now, I can do Scan.all(:item_id => @barcode) to get all the scans on a particular item, and I've got the in_station id and out_station id. What's the best way of getting the names, though, instead of ids. I assume it's gotta be easier than for every scan calling Station.get(:id=> scan.in_station).

This is easy enough using SQL, but how can I alter Scan/Station to either get the name or have a property that's a Station object, so I can do something like scan.station.name?

EDIT:

I've almost got this working. I have a Station class:

class Station
    include DataMapper::Resource

    property :id, Integer, :key => true
    property :name, String
end

and I got rid of property :in_station and property :out_station in Scan and replaced with:

belongs_to :in_station,        :model => 'Station', :child_key => 'id'
belongs_to :out_station,       :model => 'Station', :child_key => 'id'

Which I think/hope is saying "there's a field called in_station which is a foreign key into the Station table and one called out_station which is the same". Indeed, in_station and out_station are now instances of Station, BUT, they're the object. Even though in_station and out_station are different values, I'm getting the same object for each on every Scan. What am I doing wrong, how can I indicate that in_station and out_station are both references to Station but, when their ids are different, I expect different objects.

How about doing this:

class Station
  include DataMapper::Resource

  property :id, Serial
  # rest of the properties

  has n, :scans
end

class Scan
  include DataMapper::Resource

  property :id, Serial
  # rest of the properties

  belongs_to :station
end

Then you just do this to access the associated station:

station = Station.create
scan    = station.scans.create

scan.station # returns the associated station

That should work for you at match your schema.

The assumption is that we don't want to change the underlying SQL schema. So we have to tell DataMapper to use the existing foreign key names (in_station and out_station). The twist is that DataMapper will choke if the association name is the same as the child key. That's why I have the 'my_' prefix on the association names.

class Scan
  include DataMapper::Resource

  #rest of the properties

  belongs_to :my_in_station, :model => 'Station', :child_key => 'in_station'
  belongs_to :my_out_station, :model => 'Station', :child_key => 'out_station'
end

Usage

s = Scan.get(id)
s.my_in_station.name
s.my_out_station.name