Mongoose虚拟返回空值

我有个人模式:

const person = new mongoose.Schema({
    name: String,
    birthday: Date,
    sex: String
},
{
    toObject: { virtuals: true },
    toJSON: { virtuals: true }
});

person.virtual('tasks', {
    ref: 'task',
    localField: '_id',
    foreignField: 'person'
});

export default mongoose.model('person', person);

还有一个任务 person 领域:

const schema = new mongoose.Schema({

    person: {
        type: mongoose.Schema.Types.ObjectId,
        ref: 'person',
        required: true
    },

    name: String
});

export default mongoose.model('task', schema);

我的API检索一个人:

api.get('/person/:id', async (req, res) => {
    let personID = req.params.id;

    if (personID) {
        let person = await Person.findById(personID);

        if (person)
            res.json(person);
        else
            res.status(404).end();
    }
    else {
        res.status(400).end();
    }
});

当我为一个人查询API时, tasks 总是 null 有什么想法吗?

谢谢

更新

我尝试过:

let person = await Person.findById(personID).populate('tasks').exec();

它还在返回 无效的 .