1 files changed, 30 insertions, 0 deletions
diff --git a/lib/arel/algebra/relations/utilities/compound.rb b/lib/arel/algebra/relations/utilities/compound.rb
new file mode 100644
index 0000000000..5e775618f1
--- /dev/null
+++ b/lib/arel/algebra/relations/utilities/compound.rb
@@ -0,0 +1,30 @@
+module Arel
+  class Compound < Relation
+    attr_reader :relation
+    delegate :joins, :join?, :inserts, :taken, :skipped, :name, :externalizable?,
+             :column_for, :engine,
+             :to => :relation
+
+    [:attributes, :wheres, :groupings, :orders].each do |operation_name|
+      class_eval <<-OPERATION, __FILE__, __LINE__
+        def #{operation_name}
+          @#{operation_name} ||= relation.#{operation_name}.collect { |o| o.bind(self) }
+        end
+      OPERATION
+    end
+
+    def hash
+      @hash ||= :relation.hash
+    end
+
+    def eql?(other)
+      self == other
+    end
+
+  private
+
+    def arguments_from_block(relation, &block)
+      block_given?? [yield(relation)] : []
+    end
+  end
+end