diff --git a/lib/groupdate/active_record.rb b/lib/groupdate/active_record.rb
index 87d7be8..239d07d 100644
--- a/lib/groupdate/active_record.rb
+++ b/lib/groupdate/active_record.rb
@@ -1,6 +1,7 @@
 require "active_record"
 require "groupdate/order_hack"
 require "groupdate/scopes"
+require "groupdate/calculations"
 require "groupdate/series"
 
 ActiveRecord::Base.send(:extend, Groupdate::Scopes)
diff --git a/lib/groupdate/calculations.rb b/lib/groupdate/calculations.rb
new file mode 100644
index 0000000..c7d8a7b
--- /dev/null
+++ b/lib/groupdate/calculations.rb
@@ -0,0 +1,26 @@
+module Groupdate
+  class Calculations
+    attr_reader :relation
+
+    def initialize(relation)
+      @relation = relation
+    end
+
+    def include?(method)
+      # https://github.com/rails/rails/blob/master/activerecord/lib/active_record/relation/calculations.rb
+      ActiveRecord::Calculations.method_defined?(method) || custom_calculations.include?(method)
+    end
+
+    def custom_calculations
+      return [] if !model.respond_to?(:groupdate_calculation_methods)
+      model.groupdate_calculation_methods
+    end
+
+    private
+
+    def model
+      return if !relation.respond_to?(:klass)
+      relation.klass
+    end
+  end
+end
diff --git a/lib/groupdate/series.rb b/lib/groupdate/series.rb
index b1c9958..ac11e06 100644
--- a/lib/groupdate/series.rb
+++ b/lib/groupdate/series.rb
@@ -27,29 +27,4 @@ module Groupdate
       nil
     end
   end
-
-  class Calculations
-    attr_reader :relation
-
-    def initialize(relation)
-      @relation = relation
-    end
-
-    def include?(method)
-      # https://github.com/rails/rails/blob/master/activerecord/lib/active_record/relation/calculations.rb
-      ActiveRecord::Calculations.method_defined?(method) || custom_calculations.include?(method)
-    end
-
-    def custom_calculations
-      return [] if !model.respond_to?(:groupdate_calculation_methods)
-      model.groupdate_calculation_methods
-    end
-
-    private
-
-    def model
-      return if !relation.respond_to?(:klass)
-      relation.klass
-    end
-  end
 end
diff --git a/test/test_helper.rb b/test/test_helper.rb
index 5b3984b..a167b81 100644
--- a/test/test_helper.rb
+++ b/test/test_helper.rb
@@ -361,9 +361,9 @@ module TestDatabase
   # custom model calculation methods
 
   def test_custom_model_calculation_method
-    create_user "2014-05-01", 1
-    create_user "2014-05-01", 2
-    create_user "2014-05-03", 3
+    create_user "2014-05-01"
+    create_user "2014-05-01"
+    create_user "2014-05-03"
 
     expected = {
       Date.parse("2014-05-01") => 2,
--
libgit2 0.21.0