adding maximum nodes

lib/arel/attributes/attribute.rb

@@ -16,6 +16,10 @@ def count distinct = false
       def sum
         Nodes::Sum.new [self], Nodes::SqlLiteral.new('sum_id')
       end
+
+      def maximum
+        Nodes::Max.new [self], Nodes::SqlLiteral.new('max_id')
+      end
     end
 
     class String  < Attribute; end

lib/arel/nodes.rb

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

lib/arel/nodes/max.rb

+module Arel
+  module Nodes
+    class Max
+      attr_accessor :expressions, :alias
+
+      def initialize expr, aliaz = nil
+        @expressions = expr
+        @alias       = aliaz
+      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
+end

lib/arel/visitors/to_sql.rb

@@ -76,6 +76,11 @@ def visit_Arel_Nodes_Sum o
           visit x }.join(', ')})#{o.alias ? " AS #{visit o.alias}" : ''}"
       end
 
+      def visit_Arel_Nodes_Max o
+        "MAX(#{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 @@
 module Arel
   module Attributes
     describe 'attribute' do
+      describe '#maximum' do
+        it 'should create a MAX node' do
+          relation = Table.new(:users)
+          relation[:id].maximum.should be_kind_of Nodes::Max
+        end
+
+        # FIXME: backwards compat. Is this really necessary?
+        it 'should set the alias to "max_id"' do
+          relation = Table.new(:users)
+          mgr = relation.project relation[:id].maximum
+          mgr.to_sql.should be_like %{
+            SELECT MAX("users"."id") AS max_id
+            FROM "users"
+          }
+        end
+      end
+
       describe '#sum' do
         it 'should create a SUM node' do
           relation = Table.new(:users)

spec/arel/nodes/sum_spec.rb

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