def order_expressions
      @order_expressions ||=
      [filter_parameters[:order]].flatten.map do |order_predicate|
        next if order_predicate.blank?
        expr = order_expr order_predicate
        [expr.expression, expr]
      end.compact
    end
def order_expressions
      @order_expressions ||=
      [filter_parameters[:order]].flatten.map do |order_predicate|
        next if order_predicate.blank?
        expr = order_expr order_predicate
        [expr.expression, expr]
      end.compact
    end
Associative array (not a Hash) of names to order expressions
 TODO this should just be a hash