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, 13 insertions, 0 deletions

diff --git a/activerecord/test/schema/schema.rb b/activerecord/test/schema/schema.rb
index 350113eaab..274879d4af 100644
--- a/activerecord/test/schema/schema.rb
+++ b/activerecord/test/schema/schema.rb
@@ -364,6 +364,19 @@ ActiveRecord::Schema.define do
     t.integer :follower_id
   end
 
+  create_table :games, force: true do |t|
+    t.integer :game_owner_id
+    t.integer :game_collection_id
+  end
+
+  create_table :game_boards, force: true do |t|
+    t.integer :game_id
+  end
+
+  create_table :game_collections, force: true
+
+  create_table :game_owners, force: true
+
   create_table :goofy_string_id, force: true, id: false do |t|
     t.string :id, null: false
     t.string :info