Remove undocumented feature from has_one where you could pass false as the second parameter to build_assoc or create_assoc, and the existing associated object would be untouched (the foreign key would not be nullified, and it would not be deleted). If you want behaviour similar to this you can do the following things:

* Use :dependent => :nullify (or don't specify :dependent) if you want to prevent the existing associated object from being deleted
* Use has_many if you actually want multiple associated objects
* Explicitly set the foreign key if, for some reason, you really need to have multiple objects associated with the same has_one. E.g.

    previous = obj.assoc
    obj.create_assoc
    previous.update_attributes(:obj_id => obj.id)

1 files changed, 3 insertions, 8 deletions

diff --git a/activerecord/lib/active_record/associations.rb b/activerecord/lib/active_record/associations.rb
index e7d3e45da2..bbf96f52ed 100644
--- a/activerecord/lib/active_record/associations.rb
+++ b/activerecord/lib/active_record/associations.rb
@@ -1556,20 +1556,15 @@ module ActiveRecord
 
         def association_constructor_method(constructor, reflection, association_proxy_class)
           redefine_method("#{constructor}_#{reflection.name}") do |*params|
-            attributees      = params.first unless params.empty?
-            replace_existing = params[1].nil? ? true : params[1]
-            association      = association_instance_get(reflection.name)
+            attributes  = params.first unless params.empty?
+            association = association_instance_get(reflection.name)
 
             unless association
               association = association_proxy_class.new(self, reflection)
               association_instance_set(reflection.name, association)
             end
 
-            if association_proxy_class == HasOneAssociation
-              association.send(constructor, attributees, replace_existing)
-            else
-              association.send(constructor, attributees)
-            end
+            association.send(constructor, attributes)
           end
         end