Fix read related values of reference fields

By using just only the id to store the related values, it could result a mixing
of different model with the same id. So now, it uses the reference value.
(transplanted from fb51f5b4aa4c504466cd75db60bf23bc9053209b)

trytond/model/modelsql.py

@@ -686,7 +686,7 @@ class ModelSQL(ModelStorage):
                     record2 = obj.read(cursor, user, record_id,
                             fields_related[field], context=context)
                     del record2['id']
-                    fields_related2values[field][record_id] = record2
+                    fields_related2values[field][record[field]] = record2
 
         if to_del or fields_related.keys() or datetime_fields:
             for record in res:
@@ -712,7 +712,7 @@ class ModelSQL(ModelStorage):
                                     continue
                                 record[field + '.' + related] = \
                                         fields_related2values[field][
-                                                record_id][related]
+                                            record[field]][related]
                 for field in to_del:
                     del record[field]