adding count nodes

lib/arel/nodes.rb

@@ -3,6 +3,7 @@ require 'arel/nodes/equality'
 require 'arel/nodes/or'
 
 require 'arel/nodes/in'
+require 'arel/nodes/count'
 require 'arel/nodes/sql_literal'
 require 'arel/nodes/select_core'
 require 'arel/nodes/select_statement'

lib/arel/nodes/count.rb

@@ -0,0 +1,12 @@
+module Arel
+  module Nodes
+    class Count
+      attr_accessor :expressions, :distinct
+
+      def initialize expr, distinct = false
+        @expressions = expr
+        @distinct    = distinct
+      end
+    end
+  end
+end

lib/arel/nodes/sql_literal.rb

@@ -1,6 +1,9 @@
 module Arel
   module Nodes
     class SqlLiteral < String
+      def count distinct = false
+        Count.new [self], distinct
+      end
     end
   end
 end

lib/arel/visitors/to_sql.rb

@@ -58,6 +58,10 @@ module Arel
         ].compact.join ' '
       end
 
+      def visit_Arel_Nodes_Count o
+        "COUNT(#{o.distinct ? 'DISTINCT ' : ''}#{o.expressions.map { |x| visit x }.join(', ')})"
+      end
+
       def visit_Arel_Nodes_TableAlias o
         "#{visit o.relation} #{quote_table_name o.name}"
       end

spec/arel/nodes/sql_literal_spec.rb

@@ -0,0 +1,19 @@
+module Arel
+  module Nodes
+    describe 'sql literal' do
+      describe 'count' do
+        it 'makes a count node' do
+          node = SqlLiteral.new('*').count
+          viz = Visitors::ToSql.new Table.engine
+          viz.accept(node).should be_like %{ COUNT(*) }
+        end
+
+        it 'makes a distinct node' do
+          node = SqlLiteral.new('*').count true
+          viz = Visitors::ToSql.new Table.engine
+          viz.accept(node).should be_like %{ COUNT(DISTINCT *) }
+        end
+      end
+    end
+  end
+end