[Python] ERPNext Server Script API call returns 200 but does not update database or return...

I’m trying to update Sales Order Items in ERPNext using a Server Script of type API.

I want to hit the endpoint from Postman and update custom fields (custom_buying_price, custom_qty) in the Sales Order Item child table.

Here’s my script:

def custom_update_so_items(sales_order, items=None):
"""
Update Sales Order items (qty, rate, custom fields) with validations.
Works for draft and submitted Sales Orders.
"""
try:
# Parse JSON items if string
if isinstance(items, str):
items = frappe.parse_json(items)
if not items:
return {"status": "error", "message": "Parameter 'items' is required."}

# Load Sales Order
so = frappe.get_doc("Sales Order", sales_order)
so.flags.ignore_validate_update_after_submit = True
so.flags.ignore_permissions = True
so.flags.ignore_mandatory = True

# Map child rows by docname
by_name = {d.name: d for d in so.items}

# Update fields
for it in items:
rowname = (it or {}).get("docname")
if not rowname or rowname not in by_name:
continue

row = by_name[rowname]

# Update standard fields
if it.get("qty") is not None:
row.qty = frappe.utils.flt(it["qty"])
if it.get("rate") is not None:
row.rate = frappe.utils.flt(it["rate"])

# Update custom fields
if it.get("custom_buying_price") is not None:
row.custom_buying_price = frappe.utils.flt(it["custom_buying_price"])
if it.get("custom_margin_amount") is not None:
row.custom_margin_amount = frappe.utils.flt(it["custom_margin_amount"])
if it.get("custom_margin_percentage") is not None:
row.custom_margin_percentage = frappe.utils.flt(it["custom_margin_percentage"])

# Recalculate amount
row.amount = row.qty * row.rate

# Save document
so.save(ignore_permissions=True)
frappe.db.commit()

# If custom fields still not updated, try direct DB update
for it in items:
rowname = (it or {}).get("docname")
if not rowname:
continue

updates = {}
if it.get("custom_buying_price") is not None:
updates["custom_buying_price"] = frappe.utils.flt(it["custom_buying_price"])
if it.get("custom_margin_amount") is not None:
updates["custom_margin_amount"] = frappe.utils.flt(it["custom_margin_amount"])
if it.get("custom_margin_percentage") is not None:
updates["custom_margin_percentage"] = frappe.utils.flt(it["custom_margin_percentage"])

if updates:
frappe.db.set_value("Sales Order Item", rowname, updates)

frappe.db.commit()

return {"status": "ok", "name": so.name}

except Exception as e:
frappe.log_error(frappe.get_traceback(), "custom_update_so_items_simple")
return {"status": "error", "message": str(e)}


What I tried using Postman

POST https:///api/method/update_so_items_with_custom_fields

Headers: Content-Type: application/json + Authorization Token

Body:

{
"sales_order": "Ashish-1990",
"items": [
{
"docname": "actual_docname_from_sales_order_item",
"qty": 5,
"rate": 120,
"custom_buying_price": 100,
"custom_margin_amount": 20,
"custom_margin_percentage": 16.7
}
]
}

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