Coming from an RDMS background I am trying to understand NoSQL databases and planning a simple project that includes topics, posts & comments.

Topics have posts & posts have comments

I have found the following guide that suggests using the following top-level collections:

1. A users collection
2. A posts collection
3. A user-posts collection
4. A posts-comments collection

https://firebaseopensource.com/projects/firebase/quickstart-android/database/readme/

I fail to understand the benefits of (3) above as surely we can simply filter (2) based on the user, even 3 would still need to be filtered.

What is the logic of having comments as a top-level collection as opposed to having comments as a subcollection under posts? Is this not the better way to store hierarchical data?

CodePudding user response：

In the NoSQL world, we are structuring a database according to the queries that we want to perform.

What is the logic of having comments as a top-level collection as opposed to having comments as a subcollection under posts?

None is better than the other. However, there are some differences:

• What are the benefits of using a root collection in Firestore vs. a subcollection?

Is this not the better way to store hierarchical data?

There is no "perfect", "the best" or "the correct" solution for structuring a Cloud Firestore database. We always choose to create a structure for our database that satisfies our queries. So in your case, I would create a schema that looks like this:

Firestore-root
  |
  --- users (collection)
  |    |
  |    --- $uid (document)
  |         |
  |         --- //user fields.
  |
  --- posts (collection)
       |
       --- $postId (document)
            |
            --- uid: "veryLongUid"
            |
            --- //user fields.
            |
            --- comments (sub-collection)
                  |
                  --- $commentId (document)
                         |
                         --- uid: "veryLongUid"
                         |
                         --- //comment fields.

Using this schema you can:

• Get all users.
• Get all posts in the database.
• Get all posts that correspond to only a particular user.
• Get all comments of all posts, of all users in the database. Requires a collection group query.
• Get all comments of all posts that correspond to a particular user. Requires a collection group query.
• Get all comments of all users that correspond to a particular post.
• Get all comments of a particular user that correspond to a particular post.

Am I missing something?

If you think that all the comments of a post might fit into 1 MiB maximum limitation, then you should consider adding all comments into an array. If not, I highly recommend you read the following approach:

• How to reduce Firestore costs?

Where I have explained how can we store up to billions of comments and replies in Firestore.