title: how to remove relation for specific instances with sequelize / mysql
body: <p>I created a many-to-many association between courses and users like this:</p>
  
  <pre><code>Course.belongsToMany(User, { through: 'CourseUser'});
  User.belongsToMany(Course, { through: 'CourseUser'});
  </code></pre>
  
  <p>which generates a join table called CourseUser.</p>
  
  <p>I add a relation for a specific course and user with the following server side function that happens on the click of a button:
   the userCourse variable looks like this: { UserId: 7, CourseId: 13 }</p>
  
  <pre><code>  addUser: function(req, res) {
      var userCourse = req.body;
      db.CourseUser.create(userCourse).then(function () {
        res.sendStatus(200);
      });
    }
  </code></pre>
  
  <ol>
  <li><p>Is this the correct way of adding the association between an existing user and an existing course? (if not, what is the correct way)</p></li>
  <li><p>I would like to have an ability of removing the association when clicking on another button. But I can't quite figure out how to set up the function.</p></li>
  </ol>