Firebase, multi location updates - firebase

Question

Consider the following code for atomic writes across multiple locations in FireBase:
var ref = new Firebase("https://<YOUR-FIREBASE-APP>.firebaseio.com");
var newPostRef = ref.child("posts").push();
var newPostKey = newPostRef.key();
var updatedUserData = {};
updatedUserData["users/"+authData.uid+"/posts/" + newPostKey] = true;
updatedUserData["posts/" + newPostKey] = {
title: "New Post",
content: "Here is my new post!"
};
ref.update(updatedUserData, function(error) {
if (error) {
console.log("Error updating data:", error);
}
});
This approach could be used to update the post at different locations, but how to enforce the atomic update at the server side? (via rules).
How can I make sure that the users cannot update the location /posts/ (via its direct reference) without populating the users/UID/posts/ or vice versa?

Answer 1

There are many possible such "business rules", so I'll pick one and implement that. Let's say that any post that a user refers to must exist. So you can only write to /users/myuid/posts/mypostid if /posts/mypostid exists. I'll also implement basic validation of the posts themselves.
{
"posts": {
"$postid": {
".validate": "hasChildren(['title', 'content'])",
"title": {
".validate": "newData.isString()"
},
"content": {
".validate": "newData.isString()"
},
"$other": {
".validate": false
}
}
},
"users": {
"$uid": {
"posts": {
"$postid": {
".validate": "newData.parent().parent().parent().parent().child('posts').child($postid).exists()
}
}
}
}
}
The biggest trick here is the newData.parent().parent()... bit, which ensures that we get the posts in the new data.
You have a habit of asking things like "how can I ensure that method ABC was used to update the data?", which is rarely the right way to think about matters. In the rules above I focus on validating the structure of the data and really don't care what API calls might lead to that data.