# Help needed in Domain
**URL:** https://discuss.tryton.org/t/help-needed-in-domain/1700
**Category:** Developer
**Created:** [August 30, 2019, 6:03am UTC](https://discuss.tryton.org/t/help-needed-in-domain/1700 "2019-08-30T06:03:05Z")
**Posts on this page:** 6
**Page:** 1
### Author:  [@kakamble](https://discuss.tryton.org/u/kakamble)
#### Post date: [August 30, 2019, 6:03am UTC](https://discuss.tryton.org/t/help-needed-in-domain/1700/1 "2019-08-30T06:03:05Z")
I have a model called ‘exam\_section.exam’. This model has 2 One2Many fields
```
centers = fields.One2Many('exam.centers', 'exam', 'Centers', states={
'readonly': ~Eval('state').in_(['draft'])
}, depends=['state'])
```
and
```
employees = fields.One2Many('exam.employees', 'exam', 'Employees', states={
'readonly': ~Eval('state').in_(['draft'])
}, depends=['state'])
```
In ‘exam.employees’ model, I have a Many2One field that links to ‘exam.centers’
```
center = fields.Many2One(
'exam.centers',
'Center',
domain=[()] # domain to fetch centers from centers field in 'exam_section.exam'
)
```
I want to select only those centers which are in the centers field of ‘exam\_section.exam’. What should I write in this domain?
---
### Author:  [@pokoli](https://discuss.tryton.org/u/pokoli)
#### Post date: [August 30, 2019, 6:36am UTC](https://discuss.tryton.org/t/help-needed-in-domain/1700/2 "2019-08-30T06:36:24Z")
> [@kakamble](#):
>
> What should I write in this domain?
You should write a function field that returns all the centers of the `exam_section.exam` model and use this field in a in clause:
> [(‘id’, ‘in’, Eval(‘your\_field’)]
Hope it helps
---
### Author:  [@kakamble](https://discuss.tryton.org/u/kakamble)
#### Post date: [August 30, 2019, 7:30am UTC](https://discuss.tryton.org/t/help-needed-in-domain/1700/3 "2019-08-30T07:30:35Z")
I defined a function field as follows
```
centers = fields.Function(
fields.One2Many(
'exam.centers',
'center_hidden',
'List of centers'
),
'on_change_with_centers'
)
```
and wrote ‘on\_change\_with\_centers’ as follows
```
@fields.depends('exam')
def on_change_with_centers(self, name=None):
if self.exam and self.exam.centers:
return self.exam.centers
```
But on starting the server the server gives out the following error
```
Traceback (most recent call last):
File "/home/elf9/workspace/apar_tryton/lib/python3.6/site-packages/Werkzeug-0.15.2-py3.6.egg /werkzeug/serving.py", line 302, in run_wsgi
execute(self.server.app)
File "/home/elf9/workspace/apar_tryton/lib/python3.6/site-packages/Werkzeug-0.15.2-py3.6.egg/werkzeug/serving.py", line 290, in execute
application_iter = app(environ, start_response)
File "/home/elf9/workspace/apar_tryton/lib/python3.6/site-packages/trytond-5.0.8-py3.6.egg/trytond/wsgi.py", line 121, in __call__
return self.wsgi_app(environ, start_response)
File "/home/elf9/workspace/apar_tryton/lib/python3.6/site-packages/trytond-5.0.8-py3.6.egg/trytond/wsgi.py", line 127, in __call__
return self.app(environ, start_response)
File "/home/elf9/workspace/apar_tryton/lib/python3.6/site-packages/trytond-5.0.8-py3.6.egg/trytond/wsgi.py", line 100, in wsgi_app
response = cls.response(data, request)
File "/home/elf9/workspace/apar_tryton/lib/python3.6/site-packages/trytond-5.0.8-py3.6.egg/trytond/protocols/jsonrpc.py", line 160, in response
response, cls=JSONEncoder, separators=(',', ':')),
File "/usr/lib/python3.6/json/ __init__.py", line 238, in dumps
**kw).encode(obj)
File "/usr/lib/python3.6/json/encoder.py", line 199, in encode
chunks = self.iterencode(o, _one_shot=True)
File "/usr/lib/python3.6/json/encoder.py", line 257, in iterencode
return _iterencode(o, 0)
File "/home/elf9/workspace/apar_tryton/lib/python3.6/site-packages/trytond-5.0.8-py3.6.egg/trytond/protocols/jsonrpc.py", line 61, in default
return marshaller(obj)
File "/usr/lib/python3.6/json/encoder.py", line 180, in default
o. __class__. __name__ )
TypeError: Object of type 'exam.centers' is not JSON serializable
```
---
### Author:  [@ced](https://discuss.tryton.org/u/ced)
#### Post date: [August 30, 2019, 7:31am UTC](https://discuss.tryton.org/t/help-needed-in-domain/1700/4 "2019-08-30T07:31:31Z")
The returned value for a One2Many Function must be a list of id not a list of instances.
---
### Author:  [@kakamble](https://discuss.tryton.org/u/kakamble)
#### Post date: [August 30, 2019, 9:13am UTC](https://discuss.tryton.org/t/help-needed-in-domain/1700/5 "2019-08-30T09:13:55Z")
It’s working now
Thanks! 🙂
---
### Author:  [@system](https://discuss.tryton.org/u/system)
#### Post date: [September 29, 2019, 9:13am UTC](https://discuss.tryton.org/t/help-needed-in-domain/1700/6 "2019-09-29T09:13:57Z")
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