title: UML Class diagram: inconsitency between 3 classes
body: <p>I seem to have a problem about a simple time tracker. The way its set up is that a user can track hit time related to a project or type in something freely. This means that a timetrack object always has a user but does not always have a project. The inconsistency can occure when I create a Timetrack object in the database connected to a user, but then connect a project that does not have a association with the user to the same time track object. </p>
  
  <p>Whats the best way to fix this. I've heard about directional association but I can't figure out how that solves the problem</p>
  
  <p>class diagram:
  <img src="https://i.stack.imgur.com/uTMcl.png" alt="class diagram TimeTracker"></p>