Dynamic upstream proxy_pass

[kay]

I'm trying to set upstream names by variables, but nginx recognizes
variables as hostnames, not upstream names.

For example:
map $cookie_backend $proxy_host {
    default     'backend1';
    '1'             'backend2';
}
... ... ...
upstream backend1 {
    server backend123:8080;
    server backend124:8080;
}
... ... ...
upstream backend2 {
    server backend223:8080;
    server backend224:8080;
}
... ... ...
location / {
    proxy_pass      http://$proxy_host;
}

nginx returns error message:
2014/04/09 14:19:51 [error] 1085#0: *1128620 backend1 could not be resolved
(3: Host not found) while sending to client, client: 192.168.1.145, server:
localhost, request: "GET / HTTP/1.1", host: "localhost"

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,249142,249142#msg-249142