I want to create a model 'Relation' which extends ActiveRecord::Base, set it's table name as 'questions_tags', and without primary key. What should I do?

class Relation < ActiveRecord::Base
  set_table_name 'questions_tags' # set table name, right?

  # how to define 'no-pk'?
  
end

UPDATE

I know use 'create_table' can solve this problem, but this is just what I want to know: What is the magic behind create_table(:id=>false)? How can I get the same effect without using create_table(:id=>false)?

ruby-on-rails activerecord
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