Added MINUS for Oracle

Aliased :minus to :except for the SelectManager

lib/arel/select_manager.rb

@@ -152,6 +152,7 @@ module Arel
     def except other
       Nodes::Except.new ast, other.ast
     end
+    alias :minus :except
 
     def with *subqueries
       if subqueries.first.is_a? Symbol

lib/arel/visitors/oracle.rb

@@ -61,6 +61,10 @@ module Arel
         "raw_rnum_ > #{visit o.expr}"
       end
 
+      def visit_Arel_Nodes_Except o
+        "( #{visit o.left} MINUS #{visit o.right} )"
+      end
+
       ###
       # Hacks for the order clauses specific to Oracle
       def order_hacks o

test/visitors/test_oracle.rb

@@ -134,6 +134,15 @@ module Arel
         end
 
       end
+
+      it 'modified except to be minus' do
+        left = Nodes::SqlLiteral.new("SELECT * FROM users WHERE age > 10")
+        right = Nodes::SqlLiteral.new("SELECT * FROM users WHERE age > 20")
+        sql = @visitor.accept Nodes::Except.new(left, right)
+        sql.must_be_like %{
+          ( SELECT * FROM users WHERE age > 10 MINUS SELECT * FROM users WHERE age > 20 )
+        }
+      end
     end
   end
 end