I have created a child table called “MRP”.
I am using that doctype’s “mrp” field as link field in Purchase Order Item’s table.
Now i want to have just mrp value as selection rather than “name” field as primary.
Name is autogenerated using expression {item_code} - {mrp}
ERPNext - v15.54.5
Link field screenshot

Customize form screenshots:
Please try customizing search field as following;
This search criteria will fetch all defined fields anywhere you are linking that doctype as Link Record.
This is working in search fields for parent form. But in child form search fields function for link field they have changed. It’s not updating in Link Field
The highlighted search criteria works in child table as well.
It will bring your desired fields visible and searchable in that link field of child table.
Yes it works when link field is itself not a child doctype.
Create a child doctype and use that doctype in a link field it won’t work.