title: django - comparing old and new field value before saving
body: <p>I have a django model, and I need to compare old and new values of field BEFORE saving.</p>
  <p>I've tried the <code>save()</code> inheritance, and <code>pre_save</code> signal. It was triggered correctly, but I can't find the list of actually changed fields and can't compare old and new values. Is there a way? I need it for optimization of pre-save actions.</p>
  <p>Thank you!</p>