Fix .new with multiple through associations

[samdec]

Summary

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.

The associated issue is: #32511

Other information

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.

[rails-bot]

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