# How to create a join using a reference field
**URL:** https://discuss.tryton.org/t/how-to-create-a-join-using-a-reference-field/6568
**Category:** Developer
**Created:** [October 5, 2023, 1:16am UTC](https://discuss.tryton.org/t/how-to-create-a-join-using-a-reference-field/6568 "2023-10-05T01:16:13Z")
**Posts on this page:** 4
**Page:** 1
### Author:  [@iehoshia](https://discuss.tryton.org/u/iehoshia)
#### Post date: [October 5, 2023, 1:16am UTC](https://discuss.tryton.org/t/how-to-create-a-join-using-a-reference-field/6568/1 "2023-10-05T01:16:13Z")
**I want to join move.shipment and shipment.id**
```
if 'move.shipment' not in tables:
shipment = Shipment. __table__ ()
tables['move.shipment'] = shipment
from_item = (from_item
.join(shipment, condition=move.shipment == shipment.id))
```
**I receive this error.**
psycopg2errorsUndefinedFunction: operator does not exist: character varying = integer
LINE 1: IN “stock\_shipment\_out” AS “k” ON (“b”“shipment” = “k”“id”
^
HINT: No operator matches the given name and argument types You might need to add explicit type casts
Also is related with this
```
@classmethod
def _where(cls, tables, withs):
Shipment = Pool().get('stock.shipment.out')
context = Transaction().context
shipment = Shipment. __table__ ()
move = tables['move']
where = super()._where(tables, withs)
customer = context.get('customer')
if customer:
where &= shipment.customer == customer
return where
```
I receive this error
psycopg2errorsUndefinedTable: missing FROM-clause entry for table “k”
LINE 1: “b”“effective\_date” \<= ‘2023-10-04’::date)) AND (“k”"custo
---
### Author:  [@maxx](https://discuss.tryton.org/u/maxx)
#### Post date: [October 5, 2023, 7:44am UTC](https://discuss.tryton.org/t/how-to-create-a-join-using-a-reference-field/6568/2 "2023-10-05T07:44:50Z")
move.shipment is a tuple with: shipment\_model, id
Try something like this:
```auto
from_item = (from_item
.join(shipment, (Concat(Shipment. __name__ + ',', shipment.id)) == move.shipment))
```
---
### Author:  [@iehoshia](https://discuss.tryton.org/u/iehoshia)
#### Post date: [October 5, 2023, 4:08pm UTC](https://discuss.tryton.org/t/how-to-create-a-join-using-a-reference-field/6568/3 "2023-10-05T16:08:51Z")
It works like a charm! Thanks
---
### Author:  [@system](https://discuss.tryton.org/u/system)
#### Post date: [October 5, 2023, 7:45pm UTC](https://discuss.tryton.org/t/how-to-create-a-join-using-a-reference-field/6568/4 "2023-10-05T19:45:36Z")
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