Support Attribute#asc and Attribute#desc to create orderings

lib/arel/attributes/attribute.rb

@@ -151,6 +151,14 @@ module Arel
         grouping_all :lteq, others
       end
 
+      def asc
+        Nodes::Ordering.new self, :asc
+      end
+
+      def desc
+        Nodes::Ordering.new self, :desc
+      end
+
       private
 
       def grouping_any method_id, others

lib/arel/nodes.rb

@@ -15,6 +15,7 @@ require 'arel/nodes/does_not_match'
 
 require 'arel/nodes/in'
 require 'arel/nodes/not_in'
+require 'arel/nodes/ordering'
 require 'arel/nodes/lock'
 require 'arel/nodes/function'
 require 'arel/nodes/count'

lib/arel/nodes/ordering.rb

@@ -0,0 +1,19 @@
+module Arel
+  module Nodes
+    class Ordering < Arel::Nodes::Node
+      attr_accessor :expr, :direction
+
+      def initialize expression, direction = :asc
+        @expr, @direction = expression, direction
+      end
+
+      def ascending?
+        direction == :asc
+      end
+
+      def descending?
+        direction == :desc
+      end
+    end
+  end
+end

lib/arel/visitors/dot.rb

@@ -32,6 +32,11 @@ module Arel
         visit_edge o, "expr"
       end
 
+      def visit_Arel_Nodes_Ordering o
+        visit_edge o, "expr"
+        visit_edge o, "direction"
+      end
+
       def visit_Arel_Nodes_TableAlias o
         visit_edge o, "name"
         visit_edge o, "relation"

lib/arel/visitors/to_sql.rb

@@ -107,6 +107,10 @@ module Arel
         "(#{visit o.expr})"
       end
 
+      def visit_Arel_Nodes_Ordering o
+        "#{visit o.expr} #{o.descending? ? 'DESC' : 'ASC'}"
+      end
+
       def visit_Arel_Nodes_Group o
         visit o.expr
       end

spec/attributes/attribute_spec.rb

@@ -601,6 +601,38 @@ module Arel
           }
         end
       end
+
+      describe '#asc' do
+        it 'should create an Ordering node' do
+          relation = Table.new(:users)
+          relation[:id].asc.should be_kind_of Nodes::Ordering
+        end
+
+        it 'should generate ASC in sql' do
+          relation = Table.new(:users)
+          mgr = relation.project relation[:id]
+          mgr.order relation[:id].asc
+          mgr.to_sql.should be_like %{
+            SELECT "users"."id" FROM "users" ORDER BY "users"."id" ASC
+          }
+        end
+      end
+
+      describe '#desc' do
+        it 'should create an Ordering node' do
+          relation = Table.new(:users)
+          relation[:id].desc.should be_kind_of Nodes::Ordering
+        end
+
+        it 'should generate DESC in sql' do
+          relation = Table.new(:users)
+          mgr = relation.project relation[:id]
+          mgr.order relation[:id].desc
+          mgr.to_sql.should be_like %{
+            SELECT "users"."id" FROM "users" ORDER BY "users"."id" DESC
+          }
+        end
+      end
     end
 
     describe 'equality' do

spec/visitors/to_sql_spec.rb

@@ -66,6 +66,15 @@ module Arel
         @visitor.accept(test).should be_like %{ "users"."id" = 't' }
       end
 
+      describe "Nodes::Ordering" do
+        it "should know how to visit" do
+          node = @attr.desc
+          @visitor.accept(node).should be_like %{
+            "users"."id" DESC
+          }
+        end
+      end
+
       describe "Nodes::In" do
         it "should know how to visit" do
           node = @attr.in [1, 2, 3]