Can't get Firestore Rules get() to work inside a function

Question

Firestore doesn't work well with get inside a function

I have this rule

service cloud.firestore {
  match /databases/{database}/documents {

    function isProjectOpenForAssign() {
      return get(/databases/$(database)/documents/projects/$(anyProject)).data.canAssignTask == true;
    }

    match /projects/{anyProject} {
      allow create: if request.auth != null;

      match /tasks/{anyTask} {
        allow create: if request.auth != null && (isProjectOpenForAssign());
      }
    }
  }
}

When running the simulator testing it I get:

Error running simulation — Error: simulator.rules line [23], column [14]. Function not found error: Name: [get].; Error: Invalid argument provided to call. Function: [get], Argument: ["||invalid_argument||"]

Answer 1

The problem is in the scope of where you define your function. Since you define isProjectOpenForAssign at the same level as this match match /projects/{anyProject}, the function won't have access to anyProject.

There are two solutions:

1. Pass anyProject as a parameter to isProjectOpenForAssign.

   function isProjectOpenForAssign(anyProject) {
     return get(/databases/$(database)/documents/projects/$(anyProject)).data.canAssignTask == true;
   }
   
   match /projects/{anyProject} {
     allow create: if request.auth != null;
   
     match /tasks/{anyTask} {
       allow create: if request.auth != null && (isProjectOpenForAssign(anyProject));
     }
   }
   

2. Define the function inside the match that declares anyProject.

   match /projects/{anyProject} {
     function isProjectOpenForAssign() {
       return get(/databases/$(database)/documents/projects/$(anyProject)).data.canAssignTask == true;
     }
   
     allow create: if request.auth != null;
   
     match /tasks/{anyTask} {
       allow create: if request.auth != null && (isProjectOpenForAssign());
     }
   }