Call to a member function real_escape_string() on boolean

try
$where['user_id'] = '1'; //value in single quote

instead of
$where['user_id'] = 1;

OR

Code:

public function get($where = array(), $fields = array(), $groupBy = array(), $offset = 0, $limit = 0)
   {  
       foreach ($where as $key => $value) {
           $this->db_slave->where($key, (string) $value); //edited
       }