Add association (<<) without committing to database

Is it possible in Rails to add an association to an existing record without immediately committing this change to the database? E.g. if I have Post has_many :tags

post.tags << Tag.first

This will commit to database immediately. I've tried other ways instead of <<, but without success (what I want is to create the association when saving the parent object). Is it possible to get behavior like when you are adding association to a new record with build?

post.tags.build name: "whatever"

I think this is kind of inconsistent in Rails, in some cases it would be useful to have an option to do this.

In other words I want

post.tags << Tag.first # don't hit the DB here!
post.save # hit the DB here!