Updating Firebase deletes already existing data - ios

Question

I'm building an app with Firebase, and I recently used this code to update a list/dictionary of "posts" inside a user dictionary. Take a look:
Firebase *baseRef = [[Firebase alloc] initWithUrl:#"https://<MY-APP>.firebaseio.com"];
NSString *userPostsPath = [NSString stringWithFormat:#"users/%#/posts", userId];
NSDictionary *postRef = #{ postId : postDescription };
[baseRef updateChildValues:#{
userPostsPath : postRef
// here I'd have a similar update
}withCompletionBlock:^(NSError *error, Firebase *ref) {
handler(error, ref);
}];
This works on the first update, but the second time I do this, all the existing posts are removed. However, changing the code to this:
Firebase *baseRef = [[Firebase alloc] initWithUrl:#"https://tonightdb.firebaseio.com"];
NSString *usersPostsPath = [NSString stringWithFormat:#"users/%#/posts/%#/", userId, postId];
[baseRef updateChildValues:#{
usersPostsPath : postDescription,
// similar update path
}withCompletionBlock:^(NSError *error, Firebase *ref) {
handler(error, ref);
}];
Updates Firebase correctly.
What is the difference between the two? How come the first didn't work?
Edit
Is it possible to do:
Firebase *baseRef = [[Firebase alloc] initWithUrl:#"https://<MY-APP>.firebaseio.com"];
NSString *newKey = [baseRef childByAutoId];
Then use that key do do an update that looks like this:
[baseRef updateChildValues:#{
[NSString stringWithFormat:#"/posts/%#/", newKey] : // something
[NSString stringWithFormat:#"/posts/%#/members/<something>", newKey] : // something
withCompletionBlock:^(NSError *error, Firebase *ref) {
handler(error, ref, task);
}];
Basically to issue multiple updates to the same path in the same request, that doesn't exist beforehand while avoiding overwrite issues

Answer 1

Your first example translate into an instruction to update:
"users/posts" = { "postid1": postDescription }
The second example translates to:
"users/posts/postid1" = postDescription
Being a computer program, the Firebase server takes a quite literal interpretation of the instructions you give it. It takes each update instruction and replaces the data at the path (the part before the =) with the value (the part after the =).
Knowing that, you can see that in the second example it will write the postDescription at users/posts/postid1. This will replace the existing description for that post, but that is probably what you had in mind.
The second example writes { "postid1": postDescription } at users/posts. This replaces the existing value at that location, so you're essentially replacing all existing posts with the new/updated one. That is probably not what you had in mind.
Update
If you're creating a new object and fanning out the key to multiple locations, you can make use of the fact that childByAutoId is a pure client-side operation:
let newRef = ref.childByAutoId()
let newKey = newRef.key
ref.childByAppendingPath("child1").childByAppendingPath(newKey).setValue("one")
ref.childByAppendingPath("child2").childByAppendingPath(newKey).setValue("two")