adding a select statment visitor

lib/arel/nodes/select_statement.rb

@@ -14,7 +14,8 @@ def initialize cores = [SelectCore.new]
 
       def initialize_copy other
         super
-        @cores = @cores.map { |x| x.clone }
+        @cores  = @cores.map { |x| x.clone }
+        @orders = @orders.map { |x| x.clone }
       end
     end
   end

lib/arel/visitors/postgresql.rb

 module Arel
   module Visitors
     class PostgreSQL < Arel::Visitors::ToSql
+      private
+      def visit_Arel_Nodes_SelectStatement o
+        if !o.orders.empty? && using_distinct_on?(o)
+          subquery        = o.dup
+          subquery.orders = []
+          subquery.limit  = nil
+          subquery.offset = nil
+
+          sql = super(subquery)
+          [
+            "SELECT * FROM (#{sql}) AS id_list",
+            "ORDER BY #{o.orders.map { |x| visit x }.join(', ')}",
+            ("LIMIT #{o.limit}" if o.limit),
+            (visit(o.offset) if o.offset),
+          ].compact.join ' '
+        else
+          super
+        end
+      end
+
+      def using_distinct_on?(o)
+        o.cores.any? do |core|
+          core.projections.any? do |projection|
+            /DISTINCT ON/ === projection
+          end
+        end
+      end
     end
   end
 end