Firestore security rules based on request query value

Question

I'm trying to secure requests to a collection to allow any single get, but only to allow list if a specific key is matched.

Database structure is like this:

projects
  project1
    name: "Project 1 name"
    board_id: "board1"
  project2
    name: "Project 2 name"
    board_id: "board2"

boards
  board1
  board2

The Firestore query I'm making from Vue:

// Only return projects matching the requested board_id

db
  .collection("projects")
  .where("board_id", "==", this.board_id)

The security rules I'd like to have would be something like this:

match /projects/{project} {
  allow get: if true // this works
  allow list: if resource.data.board_id == [** the board_id in the query **]

  // OR

  allow list: if [** the board_id in the query **] != null

I want to do this so you can list the projects in a specific board, but can't just list everything.

Is there a way to access the requested .where() in the security rules or do I need to nest my projects collection inside my boards collection and secure it that way?

Answer 1

It really depends on how you want to query data in the future. If you have no requirement to list all of the projects (irrespective of the board), then your current data model is better and can be secured by adding the allowed boards as a map {board_id: true} or (ideally) sub-collection to the /users document.

Current data model

Database

/projects/{project_id}
/boards/{board_id}
/users/{uid}/boardPermissions/{board_id}

Security rules

match /projects/{project} {
  allow list: if exists(/databases/$(database)/documents/users/$(request.auth.uid)/boardPermissions/${resource.data.board_id})

Alternative data model

If you want to totally partition your data (which is what I tend to do for many of my projects), then create the following model

Database

/boards/{board_id}/projects/{project_id}
/users/{uid}/boardPermissions/{board_id}

Security rules

match /boards/{board_id}/projects/{project_id} {
  allow list: if exists(/databases/$(database)/documents/users/$(request.auth.uid)/boardPermissions/${board_id})