1 files changed, 43 insertions, 0 deletions
diff --git a/lib/arel/algebra/primitives/expression.rb b/lib/arel/algebra/primitives/expression.rb
new file mode 100644
index 0000000000..875498c282
--- /dev/null
+++ b/lib/arel/algebra/primitives/expression.rb
@@ -0,0 +1,43 @@
+module Arel
+  class Expression < Attribute
+    attributes :attribute, :alias, :ancestor
+    deriving :==
+    delegate :relation, :to => :attribute
+    alias_method :name, :alias
+
+    def initialize(attribute, aliaz = nil, ancestor = nil)
+      @attribute, @alias, @ancestor = attribute, aliaz, ancestor
+    end
+
+    def aggregation?
+      true
+    end
+
+    def inspect
+      "<#{self.class.name} #{attribute.inspect}>"
+    end
+
+    module Transformations
+      def as(aliaz)
+        self.class.new(attribute, aliaz, self)
+      end
+
+      def bind(new_relation)
+        new_relation == relation ? self : self.class.new(attribute.bind(new_relation), @alias, self)
+      end
+
+      def to_attribute(relation)
+        Attribute.new(relation, @alias, :ancestor => self)
+      end
+    end
+    include Transformations
+  end
+
+  class Count    < Expression; end
+  class Distinct < Expression; end
+  class Sum      < Expression; end
+  class Maximum  < Expression; end
+  class Minimum  < Expression; end
+  class Average  < Expression; end
+end
+