add field 'single' and processor 'counter'

pc4-15459

lib/treasury/fields/single.rb

@@ -0,0 +1,16 @@
+module Treasury
+  module Fields
+    module Single
+      extend ActiveSupport::Concern
+
+      module ClassMethods
+        protected
+
+        def init_accessor(params)
+          @accessing_object = extract_object(params)
+          @accessing_field  = nil
+        end
+      end
+    end
+  end
+end

lib/treasury/processors/counter.rb

@@ -0,0 +1,37 @@
+module Treasury
+  module Processors
+    module Counter
+      include Single
+
+      protected
+
+      def process_insert
+        increment_current_value if satisfied?
+      end
+
+      def process_update
+        if satisfied?
+          increment_current_value unless was_counted?
+        else
+          decrement_current_value if was_counted?
+        end
+      end
+
+      def process_delete
+        decrement_current_value if was_counted?
+      end
+
+      def satisfied?
+        condition? @event.raw_data
+      end
+
+      def was_counted?
+        condition? @event.raw_prev_data
+      end
+
+      def condition?(data)
+        raise NotImplemenedError
+      end
+    end
+  end
+end