Firebase Firestore comment tree architecture

Question

I'm trying to implement a Reddit/HackerNews style tree of comments as part of a project, and am trying out Firestore as a database solution. However, I'm unsure as to the correct design reading through the docs. In a SQL database I would use numeric keys like:

0
1.0
1.1
1.1.1
0.0

to represent my tree. However, numeric keys like that seem to be a Firebase antipattern. The other route is using an actual tree in the json where a post is represented like:

{
  uid: 'A0000',
  content: 'foo',
  children: [
    {uid:..., content:..., children: []}]
}

but supposedly deep trees are bad in Firestore. As I understand it the reason deep trees are bad is that you have to fetch the whole thing, but in my case I'm not sure if that's a problem. A client fetching a post would fetch the root content node and the first 20 or so child trees. That could be a pretty big fetch, but not insanely so.

Does anyone know of a good standard way to implement this kind of structure?

Extra: Here is the more verbose expression of what the structure should look like once the client processes it.

{
  uid: 0,
  title: 'Check out this cat!',
  body: 'It\'s pretty cute! This **text** is [markdown](link), so it can have ' +
    'links and *stuff*. Yay!',
  poster: {
    uid: 0,
    name: 'VivaLaPanda',
    aviUrl: 'badlink',
  },
  posted: '2018-03-28',
  children: [{
      uid: 0,
      body: 'This is a comment, it\'s angry!',
      poster: {
        uid: 0,
        name: 'VivaLaPanda',
        aviUrl: 'badlink',
      },
      posted: '2018-03-20',
      children: [{
        uid: 0,
        body: 'This is a comment, it\'s neutral!',
        poster: {
          uid: 0,
          name: 'Steve',
          aviUrl: 'badlink',
        },
        posted: '2018-03-20',
        children: [{
          uid: 0,
          body: 'This is a comment, it\'s neutral!',
          poster: {
            uid: 0,
            name: 'Craig',
            aviUrl: 'badlink',
          },
          posted: '2018-04-10',
          children: []
        }, ]
      }, ]
    },
    {
      uid: 0,
      body: 'This is a comment, it\'s happy!',
      poster: {
        uid: 0,
        name: 'Craig',
        aviUrl: 'badlink',
      },
      posted: '2018-03-28',
      children: []
    },
  ]
};

Edit:

While I've marked this as answered because there is an answer, I'm still really interested in seeing something more elegant/efficient.

Edit2:

For posterity: I ended up deciding that any Firebase solution was hopelessly convoluted and just used DGraph for the data, with Firebase sitting in front for Auth.

Answer 1

Your children approach could be really messy if there are a lot of people commenting on eachother. A nicer approach would be the following structure for every comment:

// single comment

postUid // <- random generated by firebase
{
    postedBy: userUid
    postedTime: timestamp
    postIsChildOfUid: postUid // <- reference to an other post (optional if the comment didn't respond to another comment(top-level comment))
}

This doesn't even require nesting at all :). You can generate easily now a comment tree with this approach, but this has to be client side. But that should be easy!