title: Rails: how can I get unique values from column
body: <p>How can I get unique values from column in the table?
  For example, I have this Products table:</p>
  
  <pre><code>ID NAME CATEGORY
  1 name1 1st_cat
  2 name2 2nd_cat
  3 name3 1st_cat
  </code></pre>
  
  <p>Here I want to get only 2 values - 1st_cat and 2nd_cat:</p>
  
  <pre><code>&lt;%Products.each do |p|%&gt;
  &lt;%=p.category%&gt;
  &lt;%end%&gt;
  </code></pre>