我有一个“Department”的mongoose模型和另一个“Course”的mongoose模型。一个一对多关系。在课程文档中的department字段下保存department id。我想查询所有的部门，并在一个请求中填充所有的课程。下面是课程模式的代码

const mongoose = require('mongoose')

const courseSchema = new mongoose.Schema(
{
name:{
        type: String,
        trim: true,
        required: true
    },
 department:{
        type: mongoose.Schema.Types.ObjectId,
        required: true,
        ref: 'Department'
    }
module.exports = mongoose.model('Course', courseSchema)

这是部门的代码

const mongoose = require('mongoose')
const validator = require('validator')
const bcrypt = require('bcryptjs')
const departmentSchema = new mongoose.Schema({
 title:{
        type: String,
        required: true
    },
admin: {
        type: mongoose.Schema.Types.ObjectId,
        trim: true,
    }
departmentSchema.virtual('courses',{
    ref: 'Course',
    localField: '_id',
    foreignField: 'department'
})
const Department = mongoose.model(' Department',departmentSchema)

module.exports = Department

这是让所有部门

router.get('/departments', async (req, res)=>{
    Department.find().populate('courses').exec(function (err, departments) {
        if (err) {
            res.status(500).send(err)
        }
        res.status(200).send(departments)
    })
})

这段代码只返回部门，而不填充courses数组。