adding a sum node

lib/arel/attributes/attribute.rb

@@ -12,6 +12,10 @@ module Arel
       def count distinct = false
         Nodes::Count.new [self], distinct
       end
+
+      def sum
+        Nodes::Sum.new [self], Nodes::SqlLiteral.new('sum_id')
+      end
     end
 
     class String  < Attribute; end

lib/arel/nodes.rb

@@ -5,6 +5,7 @@ require 'arel/nodes/and'
 
 require 'arel/nodes/in'
 require 'arel/nodes/count'
+require 'arel/nodes/sum'
 require 'arel/nodes/sql_literal'
 require 'arel/nodes/select_core'
 require 'arel/nodes/select_statement'

lib/arel/nodes/sum.rb

@@ -0,0 +1,12 @@
+module Arel
+  module Nodes
+    class Sum
+      attr_accessor :expressions, :alias
+
+      def initialize expr, aliaz = nil
+        @expressions = expr
+        @alias       = aliaz
+      end
+    end
+  end
+end

lib/arel/visitors/to_sql.rb

@@ -69,6 +69,11 @@ module Arel
         }.join(', ')})#{o.alias ? " AS #{visit o.alias}" : ''}"
       end
 
+      def visit_Arel_Nodes_Sum o
+        "SUM(#{o.expressions.map { |x|
+          visit x }.join(', ')})#{o.alias ? " AS #{visit o.alias}" : ''}"
+      end
+
       def visit_Arel_Nodes_TableAlias o
         "#{visit o.relation} #{quote_table_name o.name}"
       end

spec/arel/attributes/attribute_spec.rb

@@ -3,6 +3,23 @@ require 'spec_helper'
 module Arel
   module Attributes
     describe 'attribute' do
+      describe '#sum' do
+        it 'should create a SUM node' do
+          relation = Table.new(:users)
+          relation[:id].sum.should be_kind_of Nodes::Sum
+        end
+
+        # FIXME: backwards compat. Is this really necessary?
+        it 'should set the alias to "sum_id"' do
+          relation = Table.new(:users)
+          mgr = relation.project relation[:id].sum
+          mgr.to_sql.should be_like %{
+            SELECT SUM("users"."id") AS sum_id
+            FROM "users"
+          }
+        end
+      end
+
       describe '#count' do
         it 'should return a count node' do
           relation = Table.new(:users)