title: How to use Sequelize belongsToMany associations?
body: <p>I have projects and users.</p>
  <p>A user can have many projects.</p>
  <p>A project can have multiple users.</p>
  <p>I tried to model this with a belongsToMany association.</p>
  <p>On my server I defined the associations like this:</p>
  <pre><code>user.belongsToMany(project, {
    through: 'writer_of_project'
    foreign-key: 'user'
    as: 'projects'
  });
  
  project.belongsToMany(user, {
    through: 'writer_of_project'
    foreign-key: 'project'
    as: 'writers'
  });
  </code></pre>
  <p>On my client it looks like this:</p>
  <pre><code>user: {
    id:     1,
    ...
    projects: [1,2,3]
  }
  
  project: {
    id:     1,
    ...
    writers: [1,4,5]
  }
  </code></pre>
  <p>On the server the association requires a third table to store the association and Sequelize doesn't seem to let me include the corresponding models from it.</p>
  <p>If I run a <code>project.find(1)</code> with <code>include:[user]</code> I get</p>
  <blockquote>
  <p>user is not associated with project!</p>
  </blockquote>
  <p>If I try to put the project from the example above into the update method, the users attribute is simply ignored (I expected a project.setUsers(projectUpdate.users to happen in the background).</p>
  <p><strong>What is the right way to deal with the loading and updating of these associations?</strong></p>