title: one to many - sequelize update - not removing/inserting children
body: <p>So I've been struggling for a few hours now with a one-to-many mapping update.</p>
  
  <p>I've got a <code>project</code> which has certain <code>tasks</code> (for example).
  I add and remove tasks through the frontend and send the revised object to by backend running with sequelize.</p>
  
  <p>Then I tried to update the records as follows:</p>
  
  <pre><code>return models.Project
      .findOne({
          where: { id: projectToUpdate.id },
          include: [models.Task]
      })
      .then(function (ProjectFromDb) {
          return models.sequelize
              .transaction({
                  isolationLevel: models.sequelize.Transaction.ISOLATION_LEVELS.READ_COMMITTED
              },
              function (t) {
                  return ProjectFromDb
                      .update(projectToUpdate,
                      {
                          include: [{ model: models.Task }]
                      })
                  });
      })
      .then(function (result) {
          return output.getSuccessResult(....
      })
      .catch(function (error) {
          return output.getErrorResult(....
      });
  </code></pre>
  
  <p>But this would only update the <code>Project</code></p>
  
  <p>Next I tried to update them with an additional <code>then</code> call:</p>
  
  <pre><code>.then(function (updateResult) {
          return updateResult.setTasks(projectToUpdate.Tasks, {transaction: t})
      })
  </code></pre>
  
  <p>But this would give me the result that he is trying to update the <code>Task</code> and set the <code>ProjectId</code> to <code>NULL</code> which is not possible because it is non-nullable.</p>
  
  <p>I am currently "manually" adding the <code>tasks</code> and removing them but this seems to be a silly way of using the framework.</p>
  
  <p>Can anyone tell me how to properly make this work with a one-to-many relationship without me calling <code>Tasks.createBulk</code> and <code>Tasks.destroy</code>?</p>
  
  <p><strong>EDIT TO INCLUDE MODEL</strong>
  JSON object looks like this:</p>
  
  <pre><code>{
      id: 1,
      projectName: 'nameOfTheProject',
      Tasks: [
          projectId: 1,
          name: 'taskName'
      ]
  }
  </code></pre>