title: Rails - find_by column other than "id" results in "null"
body: <p>Been trying to get this working for the past several hours, I'm probably over looking something quite simple. Basically I have a "posts" model, and one of the columns I have created is named <code>guid</code>, which is just a user definable field that gets saved to the database.</p>
  
  <p>I'm looking to <code>find_by</code> the <code>guid</code> instead of the default <code>id</code>.</p>
  
  <p>Of course I've tried <code>@post = Post.find_by_guid(params[:guid])</code> in my posts controller <code>show</code> action, but when I attempt to call it from the url (ex: mysite/post/53), in the server console it says <code>guid is NULL</code></p>
  
  <p>Webrick Console:</p>
  
  <pre><code>Processing by PostsController#show as HTML
    Parameters: {"id"=&gt;"53"}
    Post Load (0.4ms)  SELECT "posts".* FROM "posts" WHERE "posts"."guid" IS NULL LIMIT 1
  ....
  </code></pre>