Make sure that model.attribute is only set on :create?

My model Link has an attribute creator_id which contains a foreign key to my Member model table. I want to make sure that the creator_id is only "changeable" during the creation, and after it can't be changed anymore.

How can this be accomplished in the easiest way?

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http://groups.google.com/group/rubyonrails-talk/browse_frm/thread/411510544859a6b8/5427048061782e36#5427048061782e36