Referencing foreign keys with DataMapper - ruby-on-rails

Question

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.

Answer 1

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.

Answer 2

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