Force value of parent Reference field

Since 2.6, reference field can be used in One2Many so in this case the value
should be forced like for Many2One.

review2581002
(transplanted from 67acb340ba7caf7f6c801e2c7af6884e6603b614)

tryton/gui/window/view_form/model/field.py

@@ -816,6 +816,12 @@ class ReferenceField(CharField):
             return None
 
     def get(self, record, check_load=True, readonly=True, modified=False):
+        if record.parent_name == self.name:
+            if record.parent:
+                return '%s,%s' % (record.group.parent.model_name,
+                    record.parent.id)
+            else:
+                return None
         if (record.value.get(self.name)
                 and record.value[self.name][0]
                 and record.value[self.name][1] >= -1):