title: Why Do I need user id attribute?
body: <p>I am currently trying to design a social network type of website and this is the class diagram<br>
  that I have so far </p>
  
  <p><img src="https://i.stack.imgur.com/fkDxc.jpg" alt="enter image description here"></p>
  
  <p>at the moment I have userId and username in separate tables because I wanted to normalize these tables but now I am not sure why do I need the userId attribute? I have done research and a lot of similar projects have this attribute but I don't get why? if the username is already going to uniquely identify a particular user.</p>
  
  <p>By the way I am aware I have a problem with the requests table because at the moment with the attributes given I cannot identify a primary key
  Thanks</p>