当object_id是字符串时，mongoose引用不起作用

Question

我有两个mongo集合，一个用于部门，另一个用于用户

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现在我已经在部门集合中插入了一个文档，其中_id为＆＃39; DEPT0001＆＃39;但在用数据注册用户时

    var DeptSchema = new Schema({
name: {
    type: String,
    required: 'Department Name Missing.'
}},{
timestamps: true
});
DeptSchema.plugin(sequenceGenerator, {
field: '_id',
startAt: '0001',
prefix: 'DEPT'
});
module.exports = mongoose.model('departments', DeptSchema);

//User or staff schema definition
var UserSchema = new Schema({
name: {
    type: String,
    required: 'User Name Missing.'
},
role: {
    type: String,
    required: 'Role Missing.',
    enum: ['admin', 'sales']
},
passwordHash: String,
passwordSalt: String,
departmentId: {
    type: Schema.Types.ObjectId,
    required: 'Department Id Missing.',
    ref: 'departments'
}
},{
timestamps: true
});
UserSchema.plugin(sequenceGenerator, {
field: '_id',
startAt: '0001',
prefix: 'staff'
});
module.exports = mongoose.model('users', UserSchema);

邮递员

给出错误

    { name: 'rahul agarwal',
      username: 'rahul43',
      password: 'rev@888',
      departmentId: 'DEPT0001',
      role: 'admin' }

我的问题是我不应该使用自定义的_id字段作为参考，如果可行，那么为什么我不能引用它。