Q:

Firestore query permission denied

I've got a problem with structuring my security rules for Firestore. Specifically: 
db.collection("boards").whereEqualTo("roles.${me.email}", "admin") always fails with PERMISSION_DENIED error. 
What's weird, getting single documents works like a charm. From what I understand this query will not violate any of my security rules. It should always return a subcollection of a isAnyRole(resource) function. 
My security rules:
service cloud.firestore {
match /databases/{database}/documents {

function isSignedIn() {
    return request.auth != null;
}

match /boards/{board} {

    //Board has owner and 3 possible roles:
    //* admin: can do everything, like an owner, but can be removed
    //* idea_reader: has read only access to the board and its ideas
    //* idea_editor: har write access to ideas and read access to the board itself
    function roles() {
    return ['admin', 'idea_reader', 'idea_editor'];
        }

    function isOwner(rsc) {
    return request.auth.uid == rsc.data.ownerId;
  }

  function getRole(rsc) {
    return rsc.data.roles[request.auth.token.email];
  }

  function isOneOfRoles(rsc, array) {
    // Determine if the user is one of an array of roles
    return isSignedIn() && (getRole(rsc) in array);
  }

  function isAnyRole(rsc) {
    //Determine if user is any role or owner. 
    return isOwner(rsc) || isOneOfRoles(rsc, roles());
  }

  function isValidNewBoard() {
    // Valid if story does not exist and data is set correctly
    return resource == null
                && request.resource.data.ownerId == request.auth.uid
          && request.resource.data.name != null;
  }

  function isValidBoardUpdate() {
    // Valid if ownerId didn't change and called by owner of admin
    return (isOwner(resource) || isOneOfRoles(resource, ['admin']))
                    && resource.data.ownerId == request.resource.data.ownerId
                    && request.resource.data.name != null;
  }

  // Owner and admin can edit. Owner can delete. 
  allow write: if isValidNewBoard() || isValidBoardUpdate();
  allow delete: if isOwner(resource);

  // Owner and any role can read 
  allow read: if isAnyRole(resource);

  match /ideas/{idea} {
    // Any role can read ideas 
    allow read: if isAnyRole(get(/databases/$(database)/documents/boards/$(board)));

    //Owner, admin and idea_editor can edit ideas
    allow write: if isOwner(get(/databases/$(database)/documents/boards/$(board)))
                                || isOneOfRoles(get(/databases/$(database)/documents/boards/$(board)), ['admin', 'idea_editor']);
  }

}

match /{document=**} {
  allow read, write: if false;
}

}
}

A:

The problem was with the query, not security rules. It's a small tricky thing that is not well documented. So let's have a look at my query: 
db.collection("boards").whereEqualTo("roles.${me.email}", "admin")
and let's assume email is n.surname@gmail.com. Then the query looks like this: 
db.collection("boards").whereEqualTo("roles.n.surname@gmail.com", "admin")
So the Firestore first tries to access object roles, then object n inside of the roles and so on and so forth. That's why the query was denied - it was not compliant with the defined security rules. If it worked I could have query boards that I shouldn't be able to access. 
To fix that issue I had to modify query using FieldPath:
db.collection("boards").whereEqualTo(FieldPath.of("roles", me.email), ROLE_ADMIN)

