Wordpress: Multi-dimensional Array in post_meta - arrays

Question

I am trying to make a custom voting plugin for wordpress.
I want to use post_meta to store the data.
It should have two sets of informations (user_id and a timestamp) for every vote in every post.
So I set up an array with the data:
$up_vote = Array(
'ls_user_id' => $current_user->ID,
'ls_timestamp' => current_time('mysql')
);
The data inside the post_meta should be an Array of these Arrays.
To do this, I first create an empty Array, get the old data, and then push my first array inside like this:
$ls_up_votes = Array();
$ls_up_votes = get_post_meta(get_query_var('ls_id'), 'ls_up_votes');
array_push($ls_up_votes, $up_vote);
And later I store it like this:
update_post_meta(get_query_var('ls_id'), 'ls_up_votes', $ls_up_votes);
Now the problem:
Always when I try to get the data (and work with it), it gives me an Array which has only one element (sizeof() returns 1).
How can I store a multi-dimensional Array here?
What am I doing wrong?
thanks for your time! :)

Answer 1

[Updated]
I just noticed that you're using array_push; instead you should be using array_merge. Otherwise you're going to push an array into the existing array making it multi-dimensional, i.e. see note.
You'll also want to make sure you're using the correct params in get_post_meta, see below.
Make sure to use return meta as an array and not a serialized string, get_post_meta has a third paramater that should be set to TRUE if you want an the array returned.
Example:
get_post_meta( $post_id, 'ls_up_votes', TRUE );