[FIX] membership: remove membership line when deleting invoice

Apply the same behaviour as the unlink done on account.invoice.line.
This should properly be implemented with a ondelete=cascade but this is not
possible in stable version as it requires an update.

addons/membership/membership.py

@@ -488,6 +488,16 @@ class Invoice(osv.osv):
             member_line_obj.write(cr, uid, mlines, {'date_cancel': today})
         return super(Invoice, self).action_cancel(cr, uid, ids, context=context)
 
+    # TODO master: replace by ondelete='cascade'
+    def unlink(self, cr, uid, ids, context=None):
+        member_line_obj = self.pool.get('membership.membership_line')
+        for invoice in self.browse(cr, uid, ids, context=context):
+            mlines = member_line_obj.search(cr, uid,
+                    [('account_invoice_line', 'in',
+                        [l.id for l in invoice.invoice_line])])
+            member_line_obj.unlink(cr, uid, mlines, context=context)
+        return super(Invoice, self).unlink(cr, uid, ids, context=context)
+
 
 class account_invoice_line(osv.osv):
     _inherit='account.invoice.line'
@@ -520,6 +530,7 @@ class account_invoice_line(osv.osv):
                     member_line_obj.unlink(cr, uid, ml_ids, context=context)
         return res
 
+    # TODO master: replace by ondelete='cascade'
     def unlink(self, cr, uid, ids, context=None):
         """Remove Membership Line Record for Account Invoice Line
         """