title: Rails Postgres hstore. DB not being updated with changed values
body: <p>I'm trying to understand how to use hstore and it seems the database isn't updated if I try to modify a value in my hash.</p>
  
  <p>In my rails console I do</p>
  
  <pre><code>u = User.new
  u.hash_column = {'key' =&gt; 'first'}
  u.save
  </code></pre>
  
  <p>I get a message in my console
   (0.4ms)  BEGIN
    SQL (2.0ms)  UPDATE "users" SET "hash_column" = $1, "updated_at" = $2 WHERE ...
    (18.0ms) COMMIT
  and when I check in the DB the column has the correct data</p>
  
  <p>now when I try</p>
  
  <pre><code>u.hash_column['key'] = 'second'
  </code></pre>
  
  <p>the model seems correct in the console</p>
  
  <pre><code>puts u.hash_column
  </code></pre>
  
  <p>gives
      {"key"=>"second"}</p>
  
  <p>however when I try to save this
      u.save
  in my console I just get
     (0.3ms)  BEGIN
     (0.2ms)  COMMIT
  with no update statement and when I check the DB the data hasn't changed.</p>
  
  <p>if I change another attribute on the user model and save it hash_column still doesn't get updated. The only way I can change the hash_column in the database seems to be to assign to the complete hash like</p>
  
  <pre><code>u.hash_column = {'key' =&gt; 'second'}
  </code></pre>
  
  <p>is this how it is meant to work or am I doing something wrong?</p>