diff --git a/activerecord/lib/active_record/associations.rb b/activerecord/lib/active_record/associations.rb
index 714f623af3cbd6baa2d3b5cf29ece6230d9a8ca8..f3f77e21c06be50ef3fb77e7b71edfbbc9c15828 100644
--- a/activerecord/lib/active_record/associations.rb
+++ b/activerecord/lib/active_record/associations.rb
@@ -1213,7 +1213,8 @@ def has_many(name, scope = nil, options = {}, &extension)
       #   Returns the associated object. +nil+ is returned if none is found.
       # [association=(associate)]
       #   Assigns the associate object, extracts the primary key, sets it as the foreign key,
-      #   and saves the associate object.
+      #   and saves the associate object. To avoid database inconsistencies, permanently deletes an existing
+      #   associated object when assigning a new one, even if the new one isn't saved to database.
       # [build_association(attributes = {})]
       #   Returns a new object of the associated type that has been instantiated
       #   with +attributes+ and linked to this object through a foreign key, but has not