Update trackable association to optional

Author: smridge

lib/public_activity/orm/active_record/activity.rb

@@ -24,19 +24,20 @@ class Activity < ::ActiveRecord::Base
         self.table_name = PublicActivity.config.table_name
         self.abstract_class = true
 
-        # Define polymorphic association to the parent
-        belongs_to :trackable, polymorphic: true
-
         case ::ActiveRecord::VERSION::MAJOR
         when 5
           with_options(required: false) do
+            # Define polymorphic association to the parent
+            belongs_to :trackable, polymorphic: true
             # Define ownership to a resource responsible for this activity
             belongs_to :owner, polymorphic: true
             # Define ownership to a resource targeted by this activity
             belongs_to :recipient, polymorphic: true
           end
         when 6..7
           with_options(optional: true) do
+            # Define polymorphic association to the parent
+            belongs_to :trackable, polymorphic: true
             # Define ownership to a resource responsible for this activity
             belongs_to :owner, polymorphic: true
             # Define ownership to a resource targeted by this activity