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, 7 insertions, 0 deletions

diff --git a/activerecord/test/models/game_board.rb b/activerecord/test/models/game_board.rb
new file mode 100644
index 0000000000..01d082eddb
--- /dev/null
+++ b/activerecord/test/models/game_board.rb
@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+class GameBoard < ActiveRecord::Base
+  belongs_to :game
+  has_one :game_owner, through: :game
+  has_one :game_collection, through: :game
+end