Critique My Codeigniter Custom CMS Pages Model
I don't know Codeigniter so i can't comment on "is this a propper model". For my answer i'm just going to assume it is.

It's not much code so i'm going to focus on some details: 



  public function __construct()
  {
      parent::__construct();
  }


Those 5 lines do absolutely nothing. I'd remove them. If you need a constructor later, add it later.



  public function fetch_navigation()
  {
    $result = $this-&gt;db-&gt;order_by("nav_order", "asc")-&gt;where('published', 1)-&gt;get('navigation')-&gt;result_array();
    return $result;
  }


This is a pretty hard to read (113 Chars in that line). I'd go for something like this (to say with your formatting from above)

  public function fetch_navigation()
  {
      return $this-&gt;db
          -&gt;order_by("nav_order", "asc")
          -&gt;where('published', 1)
          -&gt;get('navigation')
          -&gt;result_array();
  }




$db = $this-&gt;db;


Seems unnecessary to create a local variable, those 6 extra chars shoudn't hurt. I needed to look twice to see where that variable is from and if its local because changes are made to it.



So far those are really minor notes.

Unreachable code ?

if ($query) {
  return $query;
} else {
  return false;
}


you are setting $query['modules'] = ... so even if that is null $query will at least contain

array("modules" =&gt; null); 


and that will always be true.