title: How to create a model without primary key in rails?
body: <p>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?</p>
  <pre><code>class Relation &lt; ActiveRecord::Base
    set_table_name 'questions_tags' # set table name, right?
  
    # how to define 'no-pk'?
    
  end
  </code></pre>
  <p><strong>UPDATE</strong></p>
  <hr />
  <p>I know use 'create_table' can solve this problem, but this is just what I want to know: What is the magic behind <code>create_table(:id=&gt;false)</code>? How can I get the same effect without using <code>create_table(:id=&gt;false)</code>?</p>