Field referenced '<str>' in SORTED BY or REDUCED TO  is  not  in
  the context of query

  Explanation:  The database field referenced with a SORTED BY  or
  REDUCED  TO  clause is not defined within this query.  This will
  have the effect of sorting, or projecting to  a  constant  value
  which is of little value.

  User Action:  Examine the query and ensure that  you  have  used
  the correct context variable, and field name.