Multipe DBs for Read/Write

I am trying to set up multiple databases in order to split up the read and write queries between two databases, a Read-Only Database and a Write Database.

I set up the multiple databases in settings.php:

$db_url['default'] = 'mysql://username:password@localhost/databasename';
$db_url['readonly'] = 'mysql://username:password@localhost/databasename';

And the only way I could figure out how to do this without having to go through every module and manually set the database with 'db_set_active()' is to go into the core file '/includes/database.mysql-common.inc' and modify the 'db_query' function:

function db_query($query) {
  $args = func_get_args();
  array_shift($args);
  $query = db_prefix_tables($query);
  if (isset($args[0]) and is_array($args[0])) { // 'All arguments in one array' syntax
    $args = $args[0];
  }
  _db_query_callback($args, TRUE);
  $query = preg_replace_callback(DB_QUERY_REGEXP, '_db_query_callback', $query);
	
  /* Added  by apersaud
	   Integrating multiple read/write databases
  */
  if(strpos(strtolower($query),"select") === 0){
    db_set_active('readonly');  //this will not contain any data from the master (write) database untill replication happens
  }
  else {
    db_set_active('default');
  }	
		
  return _db_query($query);
  db_set_active('default');	
}

However, it's not working correctly. Whenever I try to add new content (e.g. Story), I get an error message "The post could not be saved". The post is still created, but it's corrupted and I can't open it, and also the only row in the 'node_access' table is deleted somehow (which I think is causing some of the issues). But, I can't figure out what's causing that row to be deleted.

Any suggestions?