How to resolve Property subscribe does not exist on type 'number' error in Ionic Angular app?

Ionic Framework Ionic Angular
1

The below code is used in an Ionic 5 / Angular tutorial to update a Place object. I am trying to apply this same code to my project where I update a Conversation object:

Place Service:

private _places = new BehaviorSubject<Place[]>([
    new Place(
      'p1',
      'Manhattan Mansion',
      'In the heart of New York City.',
      'https://lonelyplanetimages.imgix.net/mastheads/GettyImages-538096543_medium.jpg?sharp=10&vib=20&w=1200',
      149.99,
      new Date('2019-01-01'),
      new Date('2019-12-31'),
      'abc'
    )
  ]);

  get places() {
    return this._places.asObservable();
  }

updatePlace(placeId: string, title: string, description: string) {
    return this.places.pipe(
      take(1),
      delay(1000),
      tap(places => {
        const updatedPlaceIndex = places.findIndex(pl => pl.id === placeId);
        const updatedPlaces = [...places];
        const oldPlace = updatedPlaces[updatedPlaceIndex];
        updatedPlaces[updatedPlaceIndex] = new Place(
          oldPlace.id,
          title,
          description,
          oldPlace.imageUrl,
          oldPlace.price,
          oldPlace.availableFrom,
          oldPlace.availableTo,
          oldPlace.userId
        );
        this._places.next(updatedPlaces);
      })
    );
  }

And here is the Place model:

export class Place {
  constructor(
    public id: string,
    public title: string,
    public description: string,
    public imageUrl: string,
    public price: number,
    public availableFrom: Date,
    public availableTo: Date,
    public userId: string
  ) {}
}

As you can see, the above updatePlace() method can be subscribed to when called.

I am trying to create an addMessageToConversation() method that can also be subscribed to, exactly like above.

Here is my code so far:

Conversation Service:

private _conversations = new BehaviorSubject<Conversation[]>([
    new Conversation(
      'conversation1',
      'user3',
      'user1',
      [
        new Message('message1', 'Test message', 'user3', new Date(2018, 0O5, 0O5, 17, 23, 42, 11)),
        new Message('message2', 'Another message', 'user1', new Date(2018, 0O6, 0O5, 17, 23, 42, 11))
      ])
  ]);

  get conversations() {
    return this._conversations.asObservable();
  }

addMessageToConversation(conversationId: string, message: string) {
    this._conversations.getValue().find(conversation => conversation.id === conversationId)
      .messages.push(
        new Message(
          Math.random().toString(),
          message,
          this.authService.userId,
          new Date(Date.now())
        ));
  }

And here are the Conversation & Message models:

export class Conversation {
    constructor(
        public id: string,
        public userId: string,
        public mechanicId: string,
        public messages: Message[]
    ) { }
}

export class Message {
    constructor(
        public id: string,
        public text: string,
        public userId: string,
        public timestamp: Date
    ) { }
}

The above code does update conversations , but I can’t subscribe to the method. Can someone please tell me what changes I need to make so that this is possible?

If I return what is in addMessageToConversation() & try to subscribe to it, I get this error message:

Property subscribe does not exist on type ‘number’