Filter posts by custom fields - wordpress

Question

I want to give users the ability to sort posts themselves using some filter links on the side, so for instance:
To sort by title: www.example.com/?orderby=title&order=asc
To sort by date: www.example.com/?orderby=date&order=asc
So I want to be able to sort posts using a custom field called "shares" that returns a number , and I use Advanced Custom Fields plugin to generate that field, but not sure how I can generate such query and more importantly, giving a link to apply it if possible.
thanks in advance.

Answer 1

Try this simple code and after making sure that it works, you need to improve it yourself (adding conditions f.e.)
add_action('pre_get_posts','order_post_filter');
function order_post_filter($query){
//then you will need to add some condition here
if (isset($_GET["orderby"]) and isset($_GET["order"])){
$order=$_GET["order"]=='DESC'?'DESC':'ASC';
$query->set('order',$order);
if ($_GET["orderby"]=='date'){
$orderby='date';
}
elseif ($_GET["orderby"]=='title'){
$orderby='title';
}
else {
$query->set('meta_key',$_GET["orderby"]);
$orderby='meta_value';
}
$query->set('orderby',$orderby);
}
}