How to order posts by a meta_key with pre_get_posts hook when value is array?

I would like to order my posts by a so called custom field. Or meta_key in other word. Meta key name is “rating”. It has a numeric value of 1..5, but it is set only for about 50% of the posts, which belongs to a given cathegory, this meta for the remaining posts will not be set.
I found out that the value of this meta key is actually an array with only 1 element, eg. [2]. So when I would like to echo the values for each posts – only for checking them and fiddling with the value type – I had to use this code in the main Loop:

echo get_post_meta(get_the_ID(),"rating")[0];

Notice the indexing at the end.

Now I have found a solution for rodering posts by the so called pre_get_posts hook:

function my_pre_get_posts( $query ) {
	if( is_admin() ) {
         return $query;
	}
   if ( isset($query->query_vars['post_type']) && $query->query_vars['post_type'] == 'event' ) {
   $query->set('orderby', 'meta_value_num');
   $query->set('meta_key', 'rating');
   $query->set('order', 'DESC');
}

 return $query;

}

add_action('pre_get_posts', 'my_pre_get_posts');

I set up the meta key and the values with this Ruby code:

server = XMLRPC::Client.new("www.example.com", "/xmlrpc.php", 80);
server.http_header_extra = {'accept-encoding' => 'identity'};
all_posts=server.call("wp.getPosts", 0, "username", "password",{"post_type" => "post", "post_status" => "published", "number" => "10000", "offset" => "0"}) rescue [];
selected_posts=all_posts.select{|z| z.dig("custom_fields").find{|z| z.dig("key")=="rating"}==nil}

selected_posts.each{|p|

  begin
    puts p["link"];
    print p["post_title"]+": ";
    rating=STDIN.readline.chomp;
  end while rating !~ /^[12345]$/ && rating!="";
  if rating !=""
    server.call("wp.editPost", 0, "username", "password",p["post_id"],{"custom_fields"=>[{"key"=>"rating","value"=>rating}]});
  end
  puts "-"*60;
  puts;
}

I have no clue why the value are stored as an array of 1 element.

Now how can I mark that I would like to use the first element of the meta_key for the ordering? Is there something like meta_index?