title: multicolumn primary keys in rails
body: <p>I'm trying to migrate a desktop application to rails (also dealing with quite old fashioned existing database). The problem is that I don't have a unique ID in one column, but it's three columns of a table that guarantee uniqueness of a record.</p>
  
  <p>Given I have three tables: </p>
  
  <pre><code>authors
    author_name,
    author_letter,
    author_nr1,
    author_nr2
    ...
  
  titles
    titel_nr,
    titel_name,
    ...
  
  author_titles
    titel_nr,
    author_letter,
    author_nr1,
    author_nr2
  </code></pre>
  
  <p>The "primary key" of authors consists of author_letter, author_nr1, author_nr2 here.</p>
  
  <p>So do I need sort of a multicolumn primary key here to have rails associations working? Or am I going in the wrong direction here?</p>