adding unary node

lib/arel/nodes.rb

 require 'arel/nodes/node'
+require 'arel/nodes/unary'
 require 'arel/nodes/binary'
 require 'arel/nodes/equality'
 require 'arel/nodes/between'

lib/arel/nodes/group.rb

 module Arel
   module Nodes
-    class Group
-      attr_accessor :expr
-
-      def initialize expr
-        @expr = expr
-      end
+    class Group < Arel::Nodes::Unary
     end
   end
 end

lib/arel/nodes/grouping.rb

 module Arel
   module Nodes
-    class Grouping < Arel::Nodes::Node
-      attr_accessor :expr
-
-      def initialize expression
-        @expr = expression
-      end
+    class Grouping < Arel::Nodes::Unary
     end
   end
 end

lib/arel/nodes/having.rb

 module Arel
   module Nodes
-    class Having
-      attr_accessor :expr
-
-      def initialize expr
-        @expr = expr
-      end
+    class Having < Arel::Nodes::Unary
     end
   end
 end

lib/arel/nodes/offset.rb

 module Arel
   module Nodes
-    class Offset
-      attr_accessor :value
-
-      def initialize value
-        @value = value
-      end
+    class Offset < Arel::Nodes::Unary
+      alias :value :expr
     end
   end
 end

lib/arel/nodes/ordering.rb

 module Arel
   module Nodes
-    class Ordering < Arel::Nodes::Node
-      attr_accessor :expr, :direction
+    class Ordering < Arel::Nodes::Binary
+      alias :expr :left
+      alias :direction :right
 
-      def initialize expression, direction = :asc
-        @expr, @direction = expression, direction
+      def initialize expr, direction = :asc
+        super
       end
 
       def ascending?

lib/arel/nodes/unary.rb

+module Arel
+  module Nodes
+    class Unary < Arel::Nodes::Node
+      attr_accessor :expr
+
+      def initialize expr
+        @expr = expr
+      end
+    end
+  end
+end

lib/arel/visitors/to_sql.rb

@@ -97,7 +97,7 @@ def visit_Arel_Nodes_Having o
       end
 
       def visit_Arel_Nodes_Offset o
-        "OFFSET #{visit o.value}"
+        "OFFSET #{visit o.expr}"
       end
 
       # FIXME: this does nothing on SQLLite3, but should do things on other

test/visitors/test_depth_first.rb

@@ -130,6 +130,9 @@ def test_insert_statement
         @visitor.accept stmt
         assert_equal [:a, :b, stmt.columns, :c, stmt], @collector.calls
       end
+
+      def test_offset
+      end
     end
   end
 end