我创建了您的架构并填充了一些虚假数据:

I created your schemas and populate with some fake data:

  let task1 = new Task({
    name: 'task1',
    type: 'type1',
    percentage: '10'
  });
  task1.save();
  let task2 = new Task({
    name: 'task2',
    type: 'type2',
    percentage: '20'
  });
  task2.save();
  let task3 = new Task({
    name: 'task3',
    type: 'type3',
    percentage: '30'
  });
  task3.save();

我在 tasksAssigned 字段中为该用户添加了两个任务(task1和task3):

I added two tasks(task1 and task3) for this user in the field tasksAssigned:

let user1 = new User({
    username: 'name teste',
      password: '123456',
    firstName: 'first name test',
    lastName: 'last name test',
    friends: [],
    tasksAssigned: ['5b579e94454cb206f6ca338f','5b579e94454cb206f6ca3391'],
    tasksCompleted: []});
  user1.save();

并执行了您的代码.之后，我只发现一个问题，当您调用Task.find时，您需要检查是否找到了用户，如果不检查，您会在user.tasksAssigned行中收到错误消息. /p>

And executed your code. After that I found only one problem, when you call Task.find you need to check if the user was found, if you don't check you will receive a error in the user.tasksAssigned line.

User.findById('5b579ee41ac34e0763324fe3')
    .then(user => {
      if(user) {
        Task.find({_id: {$nin: user.tasksAssigned}}).then(tasks => {
          console.log(tasks);
          res.send(tasks);
        });
      }
    })
    .catch(err => {
      console.log('error');
      console.log(err);
      res.status(500).send({
        message:
        err.message ||
        "Some error occurred while retrieving tasks not assigned to the user."
      });
    });

这是Task then方法内的控制台日志:

This is the console log inside the Task then method:

这是浏览器中路线的结果:

Here the result of the route in the browser:

在此链接中，您可以看到有关承诺的猫鼬文档:猫鼬的承诺

In this link you can see the Mongoose documentation about promises: Mongoose Promises