ActiveRecord question

The context:
project HABTM tags
tags HABTM project

In a controller i do the following
@project =
@tag =

So I have now a new project and a new tag, but nothing is created in the

When I do @project.tags << @tag the tag is added to the database. I
don’t want this behaviour.

What I want is a @project with @tags inside but nothing into the
Data should be saved only when I do
How can i do ?