link_to() fails if underlying db record does not have an "id" field (Rails 3.0.1, Ruby 1.9.2)

link_to() fails with:

No route matches {:action=>"edit", :controller=>"agents", :id=>#}

Even though set_primary_key is defined:

class Agent < ActiveRecord::Base
set_primary_key = 'a_id' end

Workaround: one can set id prior to calling and all will work:

agent.id = agent.a_id
link_to 'Edit', edit_agent_path(@agent)

This is with Rails 3.0.1, Ruby 1.9.2