title: Why use the uppercase key when creating association model in Sequelize?
body: <p>I have two model User and Post, defined the associated model as below:</p>
  
  <pre><code>Post.belongsTo(models.User)
  </code></pre>
  
  <p>It means Post model belongs to User as well as User has many Post. So the posts table in database must have the key userId for this association.</p>
  
  <p>I create the Post assigned to user using this code:</p>
  
  <pre><code>Post.create({
      UserId: 1,
  })
  </code></pre>
  
  <p>It would insert the right data to database.</p>
  
  <p>But It's not work as I change the code:</p>
  
  <pre><code>Post.create({
      userId: 1,
  })
  </code></pre>
  
  <p>Why the model use the uppercase UserId instead of userId ?</p>