I'm closing this because we had no feedback for 1 month, please reopen with the full details if you don't agree.

Which is the SQL contraint? Why it breaks? Why is this related to ModifyHeader?
Could you please share the full details of your issue.

msg61624

We detected that ModifyHEAD try to write a readonly field because we have a SQL constain. When you write, in this case, ModifyHEAD not need to write a field that is readonly defined in the model. (readonly = user could not change the value)

Why not? We do not enforce readonly on server side (until issue4207 is implemented).
There is any issue with it

It will be great if you can explain your problems. You marked this as bug so in order to other fix it we should know the problem.

See that "number_external" is a readonly. Why write it (_save_value) when is readonly?

I do not understand the issue.
The wizard sets by default the current values so no modification should be done.

Could please you explain which is the problem?

Module: sale

We see that we has a fields that is readonly, when change the header, try to write, when is readonly.

Field:

number_external = fields.Char('External Number', readonly=True)

The self.start._save_values to write are:

{'add_rules': True, 'agent': None, 'carrier': 13, 'comment': '', 'company': 1, 'coupon': '', 'currency': 50, 'description': '', 'esale': False, 'esale_coupon': '', 'external_discount': None, 'external_shipment_amount': None, 'external_tax_amount': None, 'external_total_amount': None, 'external_untaxed_amount': None, 'galatea_user': None, 'invoice_address': 1344, 'invoice_method': 'shipment', 'invoice_state': 'none', 'lines': [], 'moves': [], 'mse_code': '', 'number': '', 'number_external': '', 'origin': None, 'party': 1, 'payment_term': 2, 'payment_type': 19, 'payments': [], 'price_list': 1, 'reference': '', 'sale_date': None, 'sale_device': None, 'sale_discount': Decimal('0.0'), 'shipment_address': 1, 'shipment_comment': '', 'shipment_cost_method': 'order', 'shipment_method': 'order', 'shipment_party': None, 'shipment_state': 'none', 'shop': 1, 'state': 'draft', 'status': '', 'status_history': '', 'warehouse': 4}

See that "number_external" is in the dict values to write.