// 创建一个人物数据结构
let personSchema = mongoose.Schema({
        _id:Number,
        name:String,
        post:[{
            type:Number,
            ref:'postModel'
        }]
});

console.log(mongoose.Schema.ObjectId);
// 根据结构生成人物模型
let personModel = mongoose.model('person',personSchema);

let ObjectId = mongoose.Types.ObjectId;

// 创建一个文章数据结构
let postSchema = mongoose.Schema({
        _id:Number,
        title:String,
        author:{
            type:Number,
            ref:'personModel'
        },
        content:String
});

// 创建文章模型
let postModel = mongoose.model('post',postSchema);

Start query

postModel.find({})
    .populate('people')
    .exec((err,data)=>{
        console.log(data)
    });
    

What is the reason for this?
I changed the query using ObjectId and got the same result, mongoose version 4.10.6, node version 8.1.0