title: Get only a unique value on dates of an array
body: <p>I have trouble for what I thought will be a simple task...
  I need to remove the duplicate value of my array for example there are 3 times 2018-02-20 13:48:44 and I only want 1 of those value.</p>
  
  <pre><code>MyTab = [#&lt;Poll id: nil, created_at: "2018-02-20 13:47:15"&gt;, #&lt;Poll id: nil, created_at: "2018-02-20 13:48:44"&gt;, #&lt;Poll id: nil, created_at: "2018-02-20 13:48:44"&gt;, #&lt;Poll id: nil, created_at: "2018-02-20 13:48:44"&gt;, #&lt;Poll id: nil, created_at: "2018-02-20 14:25:14"&gt;, #&lt;Poll id: nil, created_at: "2018-02-20 14:25:14"&gt;, #&lt;Poll id: nil, created_at: "2018-02-20 14:25:14"&gt;, #&lt;Poll id: nil, created_at: "2018-02-20 14:32:38"&gt;, #&lt;Poll id: nil, created_at: "2018-02-20 14:36:42"&gt;, #&lt;Poll id: nil, created_at: "2018-02-20 14:36:42"&gt;, #&lt;Poll id: nil, created_at: "2018-02-20 14:40:16"&gt;, #&lt;Poll id: nil, created_at: "2018-02-20 15:30:15"&gt;, #&lt;Poll id: nil, created_at: "2018-02-20 15:30:15"&gt;, #&lt;Poll id: nil, created_at: "2018-03-06 10:24:25"&gt;, #&lt;Poll id: nil, created_at: "2018-03-06 10:24:25"&gt;] 
  </code></pre>
  
  <p>I already tried this but it does not work : </p>
  
  <pre><code>MyTab.uniq{|x| x.created_at}
  </code></pre>
  
  <p>Thanks for answers :).</p>
  
  <p>Z.</p>