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POReferencing foreign keys with DataMapper
 

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    <p>I've been looking through this:</p> <p><a href="http://datamapper.org/docs/find" rel="nofollow">http://datamapper.org/docs/find</a></p> <p>But haven't been able to gleam what I'm looking for, though I know it's quite simple.</p> <p>I have two tables, scans and stations, with the relevant fields:</p> <pre><code>STATIONS - id (primary key), name SCANS - id (primary key), item_id, in_station, out_station </code></pre> <p>Where <code>in_station</code> and <code>out_station</code> are foreign keys to the <code>id</code> field in the <code>stations</code> table.</p> <p>I have a <code>Scan</code> object</p> <pre><code>class Scan include DataMapper::Resource property :id, Integer, :key =&gt; true property :item_id, Integer property :in_station, Integer property :out_station, Integer end </code></pre> <p>So right now, I can do <code>Scan.all(:item_id =&gt; @barcode)</code> to get all the scans on a particular item, and I've got the <code>in_station</code> id and <code>out_station</code> id. What's the best way of getting the <em>names</em>, though, instead of ids. I assume it's gotta be easier than for every scan calling <code>Station.get(:id=&gt; scan.in_station)</code>.</p> <p>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?</p> <p><em><strong>EDIT:</em></strong></p> <p>I've almost got this working. I have a Station class:</p> <pre><code>class Station include DataMapper::Resource property :id, Integer, :key =&gt; true property :name, String end </code></pre> <p>and I got rid of <code>property :in_station</code> and <code>property :out_station</code> in <code>Scan</code> and replaced with:</p> <pre><code>belongs_to :in_station, :model =&gt; 'Station', :child_key =&gt; 'id' belongs_to :out_station, :model =&gt; 'Station', :child_key =&gt; 'id' </code></pre> <p>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.</p>
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    1. COLook at the fields that were created in the database for Scan table. Like I said below, although the name 'child_key' suggests otherwise, you need to use names like I illustrate below. What your `belongs_to` lines say is "Scan belongs to a Station with an association named 'in_station' and will load the Station with an `id` that matches the Scan property named `id`." That's not what you want. You want "Scan belongs to a Station with an association named 'in_station' and will load the Station with an `id` that matches the Scan property named `in_station_id`."
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