title: Sequelize.js: How to query N:M relation
body: <p>I have the following relations in sequelize:</p>
  
  <pre><code>Location.Offers = Location.belongsToMany(JobOffer, {
    foreignKey: 'locationId',
    through: 'JobOffer_Location',
    as: 'offers',
  });
  
  JobOffer.Locations = JobOffer.belongsToMany(Location, {
    foreignKey: 'jobOfferId',
    through: 'JobOffer_Location',
    as: 'locations',
  });
  </code></pre>
  
  <p>I'm not being able to query a job offer based on its location though:</p>
  
  <pre><code>const locations = [1, 2]
  JobOffer.findAll({
    include: [{
      model: Location,
      as: 'locations',
      where: {
        locationId: {
          $in: locations
        }
      },
    }],
  })
  </code></pre>
  
  <p>Error: Only HasMany associations support include.separate</p>
  
  <p>I have tried almost any solution I could find online, but nothing seems to work. Ideas?</p>