Angular FireStore Collections inside Documents

I am trying to build a chat app in Angular 2, using Firebase FireStore Database.
I was able to create one Collection, with a Docuement for each 'message'. I used this code to retrieve the messages:

export interface MessageItem {
  message: string;
  DateTime: Date;
  User: string;
}

messagesCollection: AngularFirestoreCollection<MessageItem>;
messages: Observable<MessageItem[]>;

getChatData() {
  this.messagesCollection = this.afs.collection<MessageItem>('chat_messages');
  this.messages = this.messagesCollection.valueChanges();
}

However, because I want multiple users, I would like to create a Document for each user, which stores inside a Collection of the messages.
This complicates it, and I'm not sure how to do it exactly.
I would be happy for some guidance.