title: Sequelize add existing entry as association to newly created entry and get newly created/updated entry
body: <p>In sequelize, is there a way to add an existing entry to a newly created model and get the new model?</p>
  
  <pre><code>const Account = sequelize.define('Account')
  const User = sequelize.define('User')
  
  Account.associate = (models) =&gt; {
    Account.belongsToMany(models.User, { through: 'UserAccount' })
  }
  
  User.associate = (models) =&gt; {
    User.belongsToMany(models.Account, { through: 'UserAccount' })
  }
  
  Promise.all([User.create(), Account.create()])
    .spread((user, account) =&gt; account.addUsers([user.id])
    .then(userAccounts =&gt; {
       // Right now I have the userAccounts, but I want the updated account 
    }
  </code></pre>
  
  <p>Is there a clean way to do this, rather than save the promise of the Account creation and lookup the account again like the following:</p>
  
  <pre><code>const accountPromise = Account.create()
  Promise.all([User.create(), accountPromise])
     .spread((user, account) =&gt; account.addUsers([user.id])
     .then(() =&gt; accountPromise)
     .then((account) =&gt; Account.findById(account.id, { include: 'Users' })
     .then(account) =&gt; {
        // this is what I want
     }
  </code></pre>
  
  <p>Note that I do not wish to create the User as part of the <code>Account.create</code> method. I'm simulating a User that already exists.</p>