title: How to update the value of a single field invoking appropriate validation
body: <p>I'm making a module to allow users to update single fields on in this case, their user entity. </p>
  
  <p>The code below is an example of the method I have initially been using to get it working and test other elements of the module</p>
  
  <pre><code>global $user;
  $account = user_load($user-&gt;uid);
  $edit = (array) $account;
  $edit['field_lastname']['und'][0]['value'] = 'test';
  user_save($account, $edit);
  </code></pre>
  
  <p>However this bypasses any field validation defined elsewhere in Drupal. I don't want to reproduce any validation written elsewhere - it's not the Drupal way!</p>
  
  <p>My question is: Is there a function in Drupal 7 that can be called to update the value of a single field. I imagine such a function would clear the appropriate caches, invoke the fields validation etc. </p>
  
  <p>I am aware the solution will be totally different to my current user object based one. I just can't for the life of me find the appropriate function in the API. I wander whether the fact I am looking for a save function alone is the problem - and that there are some other necessary steps that come before.</p>
  
  <p>Any help gratefully appreciated.</p>