title: find by reference
body: <p>so i have 2 models:</p>
  
  <pre><code>create_table "holders", :force =&gt; true do |t|
   t.string   "faceid"
   t.integer  "badges_id"
   t.datetime "created_at", :null =&gt; false
   t.datetime "updated_at", :null =&gt; false
  end
  add_index "holders", ["badges_id"], :name =&gt; "index_holders_on_badges_id"
  
  create_table "badges", :force =&gt; true do |t|
   t.string   "name"
   t.text     "description"
   t.datetime "created_at",  :null =&gt; false
   t.datetime "updated_at",  :null =&gt; false
  end
  </code></pre>
  
  <p>i need 2 things:</p>
  
  <ol>
  <li><p>to get all the badges of a certain faceid holder</p></li>
  <li><p>to get all the holders of a certain badge.</p></li>
  </ol>
  
  <p>i know its really noobs question but until now i didnt work with references so i dont really understood from the literature how to make the connection.</p>