title: Role of foreign key in bidirectional relationships
body: <p>I am a little confused about designing the database for a bidirectional relationship between a customer &amp; product. </p>
  
  <p>Just to be clear a bidirectional relationship ensures that a customer is aware of his products &amp; a product is aware of it's customer.</p>
  
  <p>I have a Customer table &amp; a Product table. Product table has a foreign key of Customer table. </p>
  
  <p>To create a bidirectional relationship should a Customer table also contain a foreign key of Product table? I've seen ORM <a href="http://fruzenshtein.com/bidirectional-many-to-one-association/" rel="nofollow">examples</a> where just one FK reference is OK for a bidirectional relationship but still a little confused, can anyone enlighten me on this?</p>
  
  <p>What difference would it make if I put a FK of Product table in the Customer table?</p>