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

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