Firestore file system optimization

I'm writing a basic user file system that uses Firestore for file metadata and Cloud Storage for Firebase for storing actual files. Firebase Storage isn't exclusively used because we want to add several attributes to folders that could become cumbersome considering GCS doesn't have "directories".

In this system, there are two types of items: directories and folders. To limit Firestore writes, children currently do not have a reference to their parent directories. Parent directories have an array containing each of their child IDs, and paths are built by traversing down each parent's children array. This is done to make moving files easier (basically, we switch the child ID from one directory's array to another directory's array). Otherwise, we would have to write a new parent into every child whenever a folder moves.

This is the data structure:

interface Directory {
  id: string;
  entryType: "directory";
  name: string;
  createdTimestamp?: Timestamp;
  modifiedTimestamp?: Timestamp;
  owners?: string[];
  viewers?: string[];
  editors?: string[];
  children: string[]; // contains references to File's ID
  color: string;
  icon: string;
  root: bool;

}

interface File{
 id: string;
  entryType: "file";
  name: string;
  createdTimestamp?: Timestamp;
  modifiedTimestamp?: Timestamp;
  owners?: string[];
  viewers?: string[];
  editors?: string[];
  storagePath: string; 
  storageBucket: string;
  contentType: string;
  metadata: Record<string, string>
}

However, based on my observations, large companies providing drive services tend to provide a parent reference with each of their children. These large companies are less constrained by the cost of multiple writes and I have yet to find a fatal flaw in this system (besides some inconvenience when constructing file paths), so I'm curious whether someone smarter than I am can explain why a parent reference should or should not be added? Thanks!

> To limit Firestore writes, children currently do not have a reference to their parent directories.

If the problem is related to the number of writes you perform in Firestore, then why are you not using Firebase Realtime Database? The write operations are free of charge.

> Parent directories have an array containing each of their child IDs, and paths are built by traversing down each parent's children array.

Since the Realtime Database is an entire JSON object you can simply create references to the files in Storage, using key-value pairs.