forgot to include

models/activity.rb

+class Activity
+  include Mongoid::Document
+  include Mongoid::Timestamps
+
+  field :anonymous, type: Boolean
+  field :activity_type, type: String
+  field :happend_at, type: Time
+
+  belongs_to :actor, class_name: "User", inverse_of: :activities, index: true, autosave: true
+  belongs_to :target, inverse_of: :activities, polymorphic: true, index: true, autosave: true
+
+  validates_presence_of :actor
+  validates_presence_of :target
+
+
+end