Fix .new with multiple through associations

This fixes a bug with building an object that has multiple
`has_many :through` associations through the same object.
Previously, when building the object via .new, the intermediate
object would be created instead of just being built.

Here's an example:
Given a GameBoard, that has_one Owner and Collection through Game.
The following line would cause a game object to be created in the
database.

    GameBoard.new(owner: some_owner, collection: some_collection)

Whereas, if passing only one of those associations into `.new` would
cause the Game object to be built and not created in the database.

Now the above code will only build the Game object, and not save it.

1 files changed, 5 insertions, 1 deletions

diff --git a/activerecord/lib/active_record/associations/has_one_through_association.rb b/activerecord/lib/active_record/associations/has_one_through_association.rb
index 491282adf7..019bf0729f 100644
--- a/activerecord/lib/active_record/associations/has_one_through_association.rb
+++ b/activerecord/lib/active_record/associations/has_one_through_association.rb
@@ -28,7 +28,11 @@ module ActiveRecord
             end
 
             if through_record
-              through_record.update(attributes)
+              if through_record.new_record?
+                through_record.assign_attributes(attributes)
+              else
+                through_record.update(attributes)
+              end
             elsif owner.new_record? || !save
               through_proxy.build(attributes)
             else