title: Difference between ForeignKey(User, unique=True) and OneToOneField
body: <p>What is different between <code>models.ForeignKey(Modelname, unique=True)</code> and <code>models.OneToOneField</code> in Django? </p>
  
  <p>Where should I use <code>models.OneToOneField</code> and <code>models.ForeignKey(Modelname, unique=True)</code>?</p>