Firebase Security Rules - Checking if data.hasChild and keeps failing - firebase

Question

I have a security rule that looks like this:
"user_feeds": {
"$userId": {
".indexOn": ["timestamp"],
".read": "auth != null",
"$postId" : {
".write": "auth != null && newData.parent().hasChild(newData.child('band_id').val())"
}
}
}
in my $postId sub-tree, I'm trying to check to see if the parent has a node that corresponds to the band_id value of the data that is being inserted.
The data looks like:
{
post_id: 1,
band_id: '-L-tyesfsdf13434',
timestamp: 34234234234
}
And an actual tree that these rules would correspond to look like:
{
"user_feeds": {
"userId1": {
"-L-tyesfsdf13434": true
}
}
}
I am using Ionic/Angular and here is my function that's actually trying to do the write operation:
addPostToFollowers(bandId, postId) {
// 1. Get the user's connections
this._bands.getBandFollowers(bandId).take(1).subscribe(connections => {
if(connections.$value !== null) {
delete connections['$key'];
delete connections['$exists'];
let connectionKeys = Object.keys(connections);
// 2. Go through those connections and add the post
let bandFeeds = {};
connectionKeys.forEach(connectionKey => {
bandFeeds[`/user_feeds/${connectionKey}/${postId}`] = {
post_id: postId,
band_id: bandId,
timestamp: -(+(new Date()))
}
})
// 3. Update the social feeds
this.db.object(`social`).update(bandFeeds);
}
})
}
However, when I try to run this, I get a permission denied and I have no idea why. Am I doing something wrong here?

Answer 1

Try using root.child('user_feeds').child($userId) instead of newData.parent():
"user_feeds": {
"$userId": {
".indexOn": ["timestamp"],
".read": "auth != null",
"$postId" : {
".write": "auth != null && root.child('user_feeds').child($userId)
.hasChild(newData.child('band_id').val())"
}
}
}
I suspect the newData snapshot is limited to the data at that location and parent() cannot be used to fetch data at a location higher in the database tree.

Answer 2

So here are my updated rules. I needed to reference another path correctly:
"user_feeds": {
"$userId": {
".indexOn": ["timestamp"],
"$postId" : {
".write": "auth != null && (root.child('social').child('user_band_follows').child($userId).hasChild(newData.child('band_id').val()) || root.child('social').child('user_band_follows').child($userId).hasChild(data.child('band_id').val()) || root.child('social').child('user_connections').child($userId).hasChild(auth.uid))"
}
}
}