title: Friendly_id scope issue. Numeric slug
body: <p>I'm trying to use Friendly ID to create more custom urls. However, the column I want to use for a slug is still a number. I don't want to create a separate column just for the slug, since it would be identical.</p>
  
  <p>I have a <em>Lot</em> that belongs to an <em>Auction</em>.<br>
  <em>Auctions</em> have many <em>Lots</em> (items for sale).</p>
  
  <p>A <em>Lot</em> has a <code>lot_number</code>, which is unique to an <em>Auction</em>. It's not unique throughout the entire table, though. It's basically just a way to order the lots in each auction.</p>
  
  <p>My URLs look like: /auctions/1/lots/21 (/auctions/:auction_id/lots/:id)<br>
  I want them to be: /auctions/1/lots/1 (/auctions/:auction_id/lots/:lot_number)</p>
  
  <p>I added the following to <strong>lot.rb</strong>:</p>
  
  <pre><code>extend FriendlyId
  friendly_id :lot_number
  </code></pre>
  
  <p>It almost works. It shows me a Lot with the correct lot number, but the wrong auction.<br>
  I read about scopes on the Friendly ID docs, which sounded perfect. I could scope the lot against the auction...<br>
  So I tried:</p>
  
  <pre><code>extend FriendlyId
  friendly_id :lot_number, :use =&gt; :scoped, :scope =&gt; :auction_id
  </code></pre>
  
  <p>Now I see the following error:</p>
  
  <pre><code>SQLite3::SQLException: no such column: lots.slug: SELECT  "lots".* FROM "lots"  WHERE "lots"."slug" = '1' LIMIT 1
  </code></pre>
  
  <p>Why is the SQL <code>WHERE lots.slug = 1</code>? Shouldn't it be <code>WHERE lots.auction_id = 1</code>?<br>
  Am I using the wrong syntax? Not sure where I've gone wrong.</p>
  
  <p>Any help would be appreciated. Thanks.</p>