Make child table's naming series from a field in the same doctype

Hi i am completely new to erpnext .I have a doctype “Farms” .In that doctype i created a child table “Farm Slots”. so i need to create a naming series for child table like [farm name ][slot number]###. Here [farm name ] is a field in farms doctype and [slot number] is a field in child table “Farm Slots”.

Child tables are not supposed to exist as independent entities, so naming is not applied to child tables.