Switch Arel::Relation to a module. This will allow for classes with Relation extended on them to represent relations themselves (as opposed to their instances being relations).

lib/arel/algebra/relations/operations/join.rb

@@ -1,5 +1,7 @@
 module Arel
-  class Join < Relation
+  class Join
+    include Relation
+
     attributes :relation1, :relation2, :predicates
     deriving :==
     delegate :name, :to => :relation1
@@ -60,7 +62,7 @@ module Arel
     end
   end
 
-  class Relation
+  module Relation
     def join?
       false
     end

lib/arel/algebra/relations/relation.rb

@@ -1,5 +1,5 @@
 module Arel
-  class Relation
+  module Relation
     attr_reader :count
 
     def session

lib/arel/algebra/relations/utilities/compound.rb

@@ -1,5 +1,7 @@
 module Arel
-  class Compound < Relation
+  class Compound
+    include Relation
+
     attr_reader :relation
     delegate :joins, :join?, :inserts, :taken, :skipped, :name, :externalizable?,
              :column_for, :engine, :sources, :locked, :table_alias,

lib/arel/algebra/relations/utilities/externalization.rb

@@ -12,7 +12,7 @@ module Arel
     end
   end
 
-  class Relation
+  module Relation
     def externalize
       @externalized ||= externalizable?? Externalization.new(self) : self
     end

lib/arel/algebra/relations/utilities/nil.rb

@@ -1,7 +1,7 @@
 require 'singleton'
 
 module Arel
-  class Nil < Relation
-    include Singleton
+  class Nil
+    include Relation, Singleton
   end
 end

lib/arel/engines/memory/relations/array.rb

@@ -1,5 +1,7 @@
 module Arel
-  class Array < Relation
+  class Array
+    include Relation
+
     attributes :array,  :attribute_names_and_types
     include Recursion::BaseCase
     deriving :==, :initialize

lib/arel/engines/memory/relations/compound.rb

@@ -1,5 +1,5 @@
 module Arel
-  class Compound < Relation
+  class Compound
     delegate :array, :to => :relation
 
     def unoperated_rows

lib/arel/engines/memory/relations/operations.rb

@@ -50,7 +50,7 @@ module Arel
     end
   end
 
-  class Join < Relation
+  class Join
     def eval
       result = []
       relation1.call.each do |row1|

lib/arel/engines/sql/relations/operations/join.rb

@@ -1,5 +1,5 @@
 module Arel
-  class Join < Relation
+  class Join
     def table_sql(formatter = Sql::TableReference.new(self))
       relation1.externalize.table_sql(formatter)
     end

lib/arel/engines/sql/relations/relation.rb

@@ -1,5 +1,5 @@
 module Arel
-  class Relation
+  module Relation
     @@connection_tables_primary_keys = {}
 
     def compiler

lib/arel/engines/sql/relations/table.rb

@@ -1,6 +1,6 @@
 module Arel
-  class Table < Relation
-    include Recursion::BaseCase
+  class Table
+    include Relation, Recursion::BaseCase
 
     cattr_accessor :engine, :tables
     attr_reader :name, :engine, :table_alias, :options

lib/arel/engines/sql/relations/utilities/compound.rb

@@ -1,5 +1,5 @@
 module Arel
-  class Compound < Relation
+  class Compound
     delegate :table, :table_sql, :to => :relation
 
     def build_query(*parts)

lib/arel/engines/sql/relations/utilities/nil.rb

@@ -1,5 +1,5 @@
 module Arel
-  class Nil < Relation
+  class Nil
     def table_sql(formatter = nil); '' end
     def name; '' end
   end

spec/support/model.rb

@@ -22,7 +22,9 @@ module Arel
     end
   end
 
-  class Model < Relation
+  class Model
+    include Relation
+
     attr_reader :engine, :attributes
 
     def self.build