load balancing - Setup dynamic upstream with nginx -

suppose have upstream source

upstream first { }  upstream second { }

and in server block

map $geoip_country_code $is_china {     default no;     cn yes; }

what achieve if $is_china, use different upstream

proxy_pass http://$preferred_host/;

i can't figure how nginx.

map might sufficient. have tried following?

map $geoip_country_code $preferred_host {     default first;     cn      second; }

EEE