adding "as" and to_sql to count nodes

lib/arel/attributes/attribute.rb

@@ -8,6 +8,10 @@ def eq other
       def in other
         Nodes::In.new self, other
       end
+
+      def count
+        Nodes::Count.new [self]
+      end
     end
 
     class String  < Attribute; end
@@ -17,4 +21,6 @@ class Decimal < Attribute; end
     class Float   < Attribute; end
     class Integer < Attribute; end
   end
+
+  Attribute = Attributes::Attribute
 end

lib/arel/nodes/count.rb

 module Arel
   module Nodes
     class Count
-      attr_accessor :expressions, :distinct
+      attr_accessor :expressions, :distinct, :alias
 
       def initialize expr, distinct = false
         @expressions = expr
         @distinct    = distinct
+        @alias       = nil
+      end
+
+      def as aliaz
+        self.alias = SqlLiteral.new(aliaz)
+        self
+      end
+
+      def to_sql
+        viz = Visitors::ToSql.new Table.engine
+        viz.accept self
       end
     end
   end

lib/arel/visitors/to_sql.rb

@@ -64,7 +64,9 @@ def visit_Arel_Nodes_Group o
       end
 
       def visit_Arel_Nodes_Count o
-        "COUNT(#{o.distinct ? 'DISTINCT ' : ''}#{o.expressions.map { |x| visit x }.join(', ')})"
+        "COUNT(#{o.distinct ? 'DISTINCT ' : ''}#{o.expressions.map { |x|
+          visit x
+        }.join(', ')})#{o.alias ? " AS #{visit o.alias}" : ''}"
       end
 
       def visit_Arel_Nodes_TableAlias o

spec/arel/attributes/attribute_spec.rb

@@ -3,6 +3,13 @@
 module Arel
   module Attributes
     describe 'attribute' do
+      describe '#count' do
+        it 'should return a count node' do
+          relation = Table.new(:users)
+          relation[:id].count.should be_kind_of Nodes::Count
+        end
+      end
+
       describe '#eq' do
         it 'should return an equality node' do
           attribute = Attribute.new nil, nil, nil

spec/arel/nodes/count_spec.rb

+require 'spec_helper'
+
+describe Arel::Nodes::Count do
+  describe "as" do
+    it 'should alias the count' do
+      table = Arel::Table.new :users
+      table[:id].count.as('foo').to_sql.should be_like %{
+        COUNT("users"."id") AS foo
+      }
+    end
+  end
+end