title: Update PostgreSQL hstore field with sql variable
body: <p>I have table <code>files</code> which has hstore column <code>details</code>. In my sql statement I insert data to it:</p>
  
  <pre><code>  UPDATE files SET details =  'new_users=&gt;new_users_count'::hstore where id = v_file_id;
  </code></pre>
  
  <p>but I want to update this hstore field not with string but with variable that is available in my sql statement. How can I do this?</p>