title: adding prefix to tables in config/database.php with some exception
body: <p>i've decided to merge tables of two related wbsits <code>A,B</code>  to one database , so they can share some tables </p>
  
  <p>i've added prefix <code>B_</code> to tables of website <code>B</code> so i can differ them with tables A  and avoid any conflict between tables with similar names </p>
  
  <p>so i have tow option now : </p>
  
  <p>1 - change every table name in the source code of website <code>B</code> and add <code>B_</code> prefix to it... which is going to take a very long time </p>
  
  <p>2 - use prefix option in config/database.php ... which i'm going to use </p>
  
  <p>but here is the problem :</p>
  
  <p>there are 3 tables which are shared by both websites and they don't have any prefix </p>
  
  <pre><code>1 - users
  2 - tractions 
  3 - sessions 
  </code></pre>
  
  <p>so what i want to do is make exepction in prefix for this 3 tables </p>
  
  <p>basically in the config/database.php for website B i want to have</p>
  
  <pre><code>$db['default'] = array(
      'dbprefix' =&gt; 'B_',
  );
  </code></pre>
  
  <p>but some how i want to make exeption for 3 tables like : </p>
  
  <pre><code>$exception = array('users' , 'tractions' , 'sessions' );
  
  if(in_array($table_name , $exception )
  $db['default']['dbprefix'] = '';
  </code></pre>
  
  <p>which is probebaly doable but i dont exactly know where and how </p>