Index: issue.rb
===================================================================
--- issue.rb	(revision 16970)
+++ issue.rb	(working copy)
@@ -104,6 +104,9 @@
     end
   }
 
+  before_save { @in_before_save_callback = true  }
+  after_save  { @in_before_save_callback = false }
+
   before_validation :default_assign, on: :create
   before_validation :clear_disabled_fields
   before_save :close_duplicates, :update_done_ratio_from_issue_status,
@@ -1009,16 +1012,12 @@
 
   # Returns the original tracker
   def tracker_was
-    Tracker.find_by_id(tracker_id_in_database)
+    Tracker.find_by(:id => tracker_id_was)
   end
 
-  # Returns the previous assignee whenever we're before the save
-  # or in after_* callbacks
+  # Returns the previous assignee
   def previous_assignee
-    # This is how ActiveRecord::AttributeMethods::Dirty checks if we're in a after_* callback
-    if previous_assigned_to_id = mutation_tracker.equal?(mutations_from_database) ? assigned_to_id_in_database : assigned_to_id_before_last_save
-      Principal.find_by_id(previous_assigned_to_id)
-    end
+    Principal.find_by(:id => @in_before_save_callback ? assigned_to_id_before_last_save : assigned_to_id_was)
   end
 
   # Returns the users that should be notified