[IMP] sale: Unlink the down payment on a SO if the invoice is unlinked

Purpose
=======

When we create a downpayment line of SO and when we delete Invoice of downpayment then so line for downpayment is not deleted.

Specification
=============

Remove the downpayment line in the SO if the related invoice is deleted

Task ID :33126

addons/sale/models/account_invoice.py

@@ -34,6 +34,14 @@ class AccountInvoice(models.Model):
         if fiscal_position:
             self.fiscal_position_id = fiscal_position
 
+    @api.multi
+    def unlink(self):
+        downpayment_lines = self.mapped('invoice_line_ids.sale_line_ids').filtered(lambda line: line.is_downpayment)
+        res = super(AccountInvoice, self).unlink()
+        if downpayment_lines:
+            downpayment_lines.unlink()
+        return res
+
     @api.onchange('partner_id', 'company_id')
     def _onchange_delivery_address(self):
         addr = self.partner_id.address_get(['delivery'])

addons/sale/models/sale.py

@@ -1209,7 +1209,7 @@ class SaleOrderLine(models.Model):
 
     @api.multi
     def unlink(self):
-        if self.filtered(lambda x: x.state in ('sale', 'done')):
+        if self.filtered(lambda line: line.state in ('sale', 'done') and (line.invoice_lines or not line.is_downpayment)):
             raise UserError(_('You can not remove an order line once the sales order is confirmed.\nYou should rather set the quantity to 0.'))
         return super(SaleOrderLine, self).unlink()