title: How can I create and update a subcollection using Angular Firestore?
body: <p>I have created a Collection "Person" in Firestore. I am able to create a document and fields from code.</p>
  
  <pre><code>ngOnInit() {
  this.personCol = this.afs.collection('person');
  //snapshotChanges is used to retreive the document data and other metadata, which includes the ID
  //we need ID to edit or delete a record
  this.person = this.personCol.snapshotChanges()
    .map(actions =&gt; {
     return actions.map(a =&gt; {
       const data = a.payload.doc.data() as Person;
       const id = a.payload.doc.id;
       return { id, data };
     });
    });}
  addPerson() {
  this.afs.collection('person').add({
    'name': this.name,
    'age': this.age,
  });}
  getPerson(PersonId) {
  this.personDoc = this.afs.doc('person/'+personId);
  this.singlePerson = this.personDoc.valueChanges();
  </code></pre>
  
  <p>} </p>
  
  <p>Now I want to add a list of books that the person borrows:</p>
  
  <pre><code>Person: 
  {Name: "John"
   Age: 32 
   Books: {Title: "First book", Author: "Some author"
           Title: "Second book" Author: "Another author"},
  {Name: "Anne"
   Age: 21
   Books: {Title: "How to... book", Author: "Author Name"
           Title: "Flowers book" Author: "Another author"} 
  </code></pre>
  
  <p>How can I create the <code>Books</code> subcollection using Angular? 
  How can I access <code>Books</code> subcollection to update the <code>title</code> and <code>author</code> from code?</p>