Firestore Security Rules for Query with Array Contains

Question

I have a Flutter app in which users can make posts and tag the post as belonging to a group. Posts are stored in a global collection and each has a Post.groupId field:

/posts/{postId}

Based on my Firestore security rules and queries, users are only allow to read posts if they are in the group for which the post is tagged (i.e the posts's groupId field). Approved group users are stored in:

/groups/{groupId}/users/{userId}

I could query the posts from a particular user's group like:

_firestore.collection('posts').where('groupId', isEqualTo: 'groupA')...

This above was all working properly.

I am attempting to make an improvement in which a post can be tagged in multiple groups instead of just one, so I am replacing the single Post.groupId field with a Post.groupIds array. A user should be able to read a post if he/she is a member of ANY of the groups from Post.groupIds. I attempt to read all posts tagged with a particular group with the following query from my Flutter app:

_firestore.collection('posts').where('groupIds', arrayContains: 'groupA')...

I keep receiving the following exception Missing or insufficient permissions with these security rules:

match /posts/{postId} {
    allow read: if canActiveUserReadAnyGroupId(resource.data.groupIds);
}

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

function getActiveUserId() {
    return request.auth.uid;
}

function isActiveUserGroupMember(groupId) {
    return isSignedIn() &&
            exists(/databases/$(database)/documents/groups/$(groupId)/users/$(getActiveUserId()));
}

function canActiveUserReadAnyGroupId(groupIds) {
    return groupIds != null && (
            (groupIds.size() >= 1 && isActiveUserGroupMember(groupIds[0])) ||
            (groupIds.size() >= 2 && isActiveUserGroupMember(groupIds[1])) ||
            (groupIds.size() >= 3 && isActiveUserGroupMember(groupIds[2])) ||
            (groupIds.size() >= 4 && isActiveUserGroupMember(groupIds[3])) ||
            (groupIds.size() >= 5 && isActiveUserGroupMember(groupIds[4]))
            );
}

With these security rules I can read a single post but I cannot make the above query. Is it possible to have security rules which allow me to make this query?

UPDATE 1

Added isSignedIn() and getActiveUserId() security rules functions for completeness.

UPDATE 2

Here is the error I am receiving when I attempt to execute this query with the Firestore Emulator locally:

     FirebaseError: 
Function not found error: Name: [size]. for 'list' @ L215

Line 215 corresponds to the allow read line within this rule:

match /posts/{postId} {
    allow read: if canActiveUserReadAnyGroupId(resource.data.groupIds);
}

Answer 1

If I had to guess, I'd say that groupIds isn't actually a List type object, which means that the field from the document is also not an array. If it's a string, this code won't work, since strings don't have a method called size() in the rules language.

If you aren't 100% certain what the type of field is going to be, you will need to check the type in the rule and determine what to do with it. You can use the is operator to check the type. For example, groupIds is list will be boolean true if you're actually working with one.

In your rules, you can use the debug() function to dump the value of some expression to the log. It will return the same value. So, you can say debug(groupIds) != null to both print the value and check it for null.

Answer 2

It appears Firestore does not currently support security rules for this scenario at the moment (thanks for your help tracking this down Doug Stevenson). I have come up with a mechanism to work around the limitation and wanted to share in case someone else is dealing with this issue. It requires an extra query but keeps me from having to create a Web API using the Admin SDK just to get around the security rules.

Posts are stored as follows (simplified):

/posts/{postId}
- userId
- timestamp
- groupIds[]
- message
- photo

Now I am adding an additional post references collection which just stores pointer information:

/postRefs/{postId}
- userId
- timestamp
- groupIds[]

The posts collection will have security rules which does all the validation to ensure the user is in at least one of the groups in which the post is tagged. Firestore is able to handle this properly for simple get requests, just not list requests at the moment.

Since the postRefs collection stores only ID's, and not sensitive information which may be in the post, its security rules can be relaxed such that I only verify a user is logged in. So, the user will perform post queries on the postRefs collection to retrieve a list of ordered postId's to be lazily loaded from the posts collection.

Clients add/delete posts to/from the normal posts collection and then there is a Cloud Function which copies the ID information over to the postRefs collection.