Documentation to Fetch value in Child Table for a field (#14282)

* Fetch value in Child Table for a field

Sample Script to fetch value in child table

* Update fetch value in child table field
diff --git a/erpnext/docs/user/manual/en/customize-erpnext/custom-scripts/custom-script-examples/fetch value in child table field b/erpnext/docs/user/manual/en/customize-erpnext/custom-scripts/custom-script-examples/fetch value in child table field
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+++ b/erpnext/docs/user/manual/en/customize-erpnext/custom-scripts/custom-script-examples/fetch value in child table field
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+## Example to fetch value in a child table field from master doctype
+
+
+### Sample Script to fetch expiry_date field from Batch doctype to Sales Invoice Item table
+
+Step 1: Create Custom Script for _**Sales Invoice**_ (parent) doctype
+
+Step 2: Script as below & Save
+
+```
+frappe.ui.form.on("Sales Invoice Item", "batch_no", function(frm, cdt, cdn) {
+	var d = locals[cdt][cdn];
+    	frappe.db.get_value("Batch", {"name": d.batch_no}, "expiry_date", function(value) {
+    		d.expiry_date = value.expiry_date;
+    	});
+});
+```