Exercise tracker, creating children in Mongoose

1

The add function need create subdoc under the Person for many Exercise and I get undefined and manual in mongoose is poor. Please help

var Schema = mongoose.Schema;
var childSchema  = new Schema({
    // Single nested subdocuments. Caveat: single nested subdocs only work
  // in mongoose >= 4.2.0
description:String,
   duration:Number,
   date:{ type: Date, default:  new Date() }  
});

  
var blogSchema = new Schema(
  { name:  {
    type: String,
    required: true},
   child: childSchema  
         
});

and route

app.post("/api/exercise/add", function (req, res ) {
   console.log(req.body)
  

var personId = req.body.userId
Person.findById(personId,                          
  function (err, docs) {
  if(!docs) { res.send('First add new user')}
  else{  
  docs.save(function(err, docs) {
 if (err) res.json({err:err});
  console.log(docs)
   **Problematic how to ?** 
 docs.child.create({description:req.body.description,duration:req.body.duration,date: req.body.date})
   
   res.json(docs);
 });    
  }
})
})

The output from logs looks
{ userId: ‘5b56476f0a78de4d3ed85775’,
description: ‘vvcvbhj’,
duration: ‘2’,
date: ‘08.05.2018’ }
{ _id: 5b56476f0a78de4d3ed85775, name: ‘lubodrinka’, __v: 0 }
The mongoose’s manual is crazy, but I found the second way with array declaration child: [childSchema] work and instead docs.child.create the docs.child.push