title: Sequelize query belongstomany not including child attributes
body: <p>I´m new to sequelize and trying to determine how to perform a query on a belongstomany relationship, where I need to check for the existence of the child relationship but I don´t want to bring any of its fields.</p>
  
  <pre><code>  Project.belongsToMany(models.User, {
          through: 'userproject', as: 'links', foreignKey: 'project_id', otherKey: 'user_id'
      });
  </code></pre>
  
  <p>My attempt:</p>
  
  <pre><code>  const linkedProjects = this.model.findAll({
          attributes: ['Project.*'],
          include: [{
              model: models.User,
              as: 'links',
              where: {
                  userid: user.sub
              }
          }]
      });
  </code></pre>
  
  <p>However this stil brings me the projected links data, which is useless for me except for the where part.</p>
  
  <p>What´s the best way to accomplish that without a "raw query" ? 
  I´m looking for, basically:</p>
  
  <pre><code>select p.* from project p 
  inner join userproject up on p.project_id = up.project_id 
  inner join user u on up.user_id = u.id
  where u.userid = 'xxx'
  </code></pre>