title: How to set nullable on association in Sequelize
body: <p>Lets say I have a <code>League</code> entity and a <code>User</code> entity and I want the manager (<code>User</code>) of that league to be mandatory. How can I tell <code>Sequelize</code> that there should be a not-null constraint on the <code>League.ManagerId</code> column?</p>
  
  <p>Here is how I defined my association:</p>
  
  <p><code>League.belongsTo User, {as: 'Manager'}</code></p>