[IMP] account_banking_mandate: Fill mandate in invoices if none is provided

Using same method as in upstream, mandate is filled on invoice creation if no
one is provided. This way, we don't need to install account_banking_mandate_sale
if we don't want to handle several mandates at sales level.

account_banking_mandate/__manifest__.py

@@ -7,7 +7,7 @@
 {
     'name': 'Account Banking Mandate',
     'summary': 'Banking mandates',
-    'version': '10.0.1.0.0',
+    'version': '10.0.1.1.0',
     'license': 'AGPL-3',
     'author': "Compassion CH, "
               "Tecnativa, "

account_banking_mandate/models/account_invoice.py

@@ -29,6 +29,24 @@ class AccountInvoice(models.Model):
                 res['mandate_id'] = invoice.mandate_id.id or False
         return res
 
+    @api.model
+    def create(self, vals):
+        """Fill the mandate_id from the partner if none is provided on
+        creation, using same method as upstream."""
+        onchanges = {
+            '_onchange_partner_id': ['mandate_id'],
+        }
+        for onchange_method, changed_fields in onchanges.items():
+            if any(f not in vals for f in changed_fields):
+                invoice = self.new(vals)
+                getattr(invoice, onchange_method)()
+                for field in changed_fields:
+                    if field not in vals and invoice[field]:
+                        vals[field] = invoice._fields[field].convert_to_write(
+                            invoice[field], invoice,
+                        )
+        return super(AccountInvoice, self).create(vals)
+
     # If a customer pays via direct debit, it's refunds should
     # be deducted form the next debit by default. The module
     # account_payment_partner copies payment_mode_id from invoice