How to make child table field read only?

Using below code, one can set child table field read only.


Syntax
 
    var df = frappe.meta.get_docfield("CHILD TABLE DOCTYPE","FIELD NAME", cur_frm.doc.name);
    df.read_only = 1;


example:
 Using this, we can set item_name field read-only. This will helpful if you want to make fields read-only based on some condition

  frappe.ui.form.on("Sales Order","onload", function(frm, cdt, cdn) {
    var df = frappe.meta.get_docfield("Sales Order Item","item_name", cur_frm.doc.name);
    df.read_only = 1;
});