Firebase (NoSQL): Denormalization vs Indexing

Question

Assume that I want to write a blogging app. Should I prefer one of the following two options? I would prefer to have as much "single source of truth" as possible, but I am still not sure whether that preference comes from my background in SQL.

Option 1 (Denormalization):

Posts: {
  post_1: {
    title: "hello",
    body: "hi there!",
    uid: "user_1",
    comments: {
      comment_1: {
        body: "hi I commented",
        uid: "user_2",
      },
      comment_2: {
        body: "bye I commented",
        uid: "user_2",
      },
    }
  }
}

Users: {
  user_1: {
    uid: "user_1",
    post_1: {
      title: "hello",
      body: "hi there!",
      uid: "user_1",
      comments: {
        comment_1: {
          body: "hi I commented",
          uid: "user_2",
        },
        comment_2: {
          body: "bye I commented",
          uid: "user_2",
        },
      }
    }
  }
}

Option 2 (Indexing):

Posts: {
  post_1: {
    title: "hello",
    body: "hi there!",
    uid: "user_1",
    authorName: "Richard",
    comments: {
      comment_1: true,
      comment_2: true
    }
  }
}

Users: {
  user_1: {
    uid: "user_1",
    displayName: "Richard",
    email: "[e-mail]",
    posts: {
      post_1: true
    },
    comments: {
      comment_1: true,
      comment_2: true
    }
  }
}

Comments: {
  comment_1: {
    body: "hi I commented",
    uid: "user_1",
  },
  comment_2: {
    body: "bye I commented",
    uid: "user_1",
  },
}

I think I should prefer option 2.

The main problem that I see with option 1 is that there are too many sources for one data. Let's say I want to extend the app so each post belongs to a certain category or tag. Then, I will have to write a post object under /categories/category_id in addition to /posts and /users/uid. When the post gets updated, I have to remember to modify the post object in three different places. If I go with option 2, I don't have this problem because there's only one source for data.

Am I missing anything?

References:

1. Firebase data structure and url
2. https://firebase.google.com/docs/database/web/structure-data

Answer 1

The second option is better because otherwise you would be forcing the user to download all the comments and posts (that can be a lot).

You can check in the documentation here.

And you can handle the duplication doing atomic writes across multiple locations.