write.rules

service cloud.firestore {
  match /databases/{database}/documents {
    // Incorrect solution
    match /teams/{teamId} {
      allow read: ...;
      allow write: if request.resource.data.owners[request.auth.uid] is int // Returns false on delete!
                        && isValidTeam();
    }

    // Correct solution
    match /teams/{teamId} {
      allow read: ...;
      allow create: if request.resource.data.owners[request.auth.uid] is int && isValidTeam(); // Allow new teams using the "request" object
      allow update: if isExistingOwner() && isValidTeam(); // Only the user who created the team can update it
      allow delete: if isExistingOwner(); // Pre-write owners can delete a team
    }
  }
}

function isExistingOwner() {
  return resource.data.owners[request.auth.uid] is int;
}

function isValidTeam() {
  return ...;
}