Allow order fields to be subqueries with order

Fields like last_compliance_status (on VmOrTemplate.yaml) contain
a limit and order.

This is because the column joins to a has_many and references
the most recent record.

ManageIQ/manageiq-ui-classic#5374

lib/extensions/ar_order.rb

@@ -8,6 +8,10 @@ def columns_for_distinct(columns, orders) #:nodoc:
           order_columns = orders.reject(&:blank?).map { |s|
               # Convert Arel node to string
               unless s.is_a?(String)
+                if s.kind_of?(Arel::Nodes::Ordering)
+                  s = s.expr
+                  keep_order = true
+                end
                 if s.respond_to?(:to_sql)
                   s = s.to_sql
                 else # for Arel::Nodes::Attribute
@@ -17,9 +21,14 @@ def columns_for_distinct(columns, orders) #:nodoc:
                   s = collector.value
                 end
               end
+              # If we haven't already removed the order clause,
               # Remove any ASC/DESC modifiers
-              s.gsub(/\s+(?:ASC|DESC)\b/i, "")
-               .gsub(/\s+NULLS\s+(?:FIRST|LAST)\b/i, "")
+              if keep_order
+                s
+              else
+                s.gsub(/\s+(?:ASC|DESC)\b/i, "")
+                 .gsub(/\s+NULLS\s+(?:FIRST|LAST)\b/i, "")
+              end
             }.reject(&:blank?).map.with_index { |column, i| "#{column} AS alias_#{i}" }
 
           (order_columns << super).join(", ")

spec/lib/extensions/ar_order_spec.rb

@@ -1,4 +1,16 @@
 describe "ar_order extension" do
+  # includes a has_many AND references the has many
+  # this introduces a DISTINCT to the query.
+  # rails uses limited_ids_for (which calls columns_for_distinct) to run a quick query
+  #
+  # when we use a virtual attribute in the sort (and the attribute has a descending order)
+  # the string munging gives us issues.
+  it "supports order when distinct is present for has_many virtual column" do
+    expect do
+      VmOrTemplate.includes(:disks).references(:disks).order(:last_compliance_status).first
+    end.not_to raise_error
+  end
+
   it "supports order when distinct is present for basic column" do
     expect do
       VmOrTemplate.includes(:disks).references(:disks).order(:id).first