# Fault: cannot execute nextval() in a read-only transaction **URL:** \ **Category:** Developer\ **Created:** [February 4, 2025, 2:01pm UTC](https://discuss.tryton.org/t/fault-cannot-execute-nextval-in-a-read-only-transaction/8275 "2025-02-04T14:01:30Z")\ **Posts on this page:** 12\ **Page:** 1
**Author:** ![maehder](https://discuss-cdn.tryton.org/letter_avatar_proxy/v4/letter/m/cdc98d/32.png) [@maehder](https://discuss.tryton.org/u/maehder)\ **Post date:** [February 4, 2025, 2:01pm UTC](https://discuss.tryton.org/t/fault-cannot-execute-nextval-in-a-read-only-transaction/8275/1 "2025-02-04T14:01:30Z")
Hi there and thank you in advance. I try to implement a simple template system for analyses in my laboratory module. The goal is to define a function that creates ‘parameter’ records that are linked to my ‘analysis’ record via One2Many when a template is chosen. The ‘Analysis’ model looks like this. ```auto class Analysis(ModelSQL, ModelView): 'Analysis' __name__ = 'toxicology.analysis' name = fields.Char('Analysis Name') analysis_template = fields.Many2One('toxicology.analysis_template', 'Analysis Template') parameter = fields.One2Many('toxicology.parameter', 'analysis', 'Parameter') ``` The function that is supposed to work looks like this: ```auto @fields.depends('analysis_template') def on_change_with_parameter(self): if self.analysis_type: pool = Pool() Parameter = pool.get('toxicology.parameter') parameters = [] for parameter in self.analysis_template.parameter: parameters.append({ 'analysis': self.id, 'analyte': parameter.analyte, 'result': parameter.result, 'uom': parameter.uom and parameter.uom.id, }) Parameter.create(parameters) ``` I do however end up with the following error message “Fault: cannot execute nextval() in a read-only transaction” which indicates that this is approach is not meant to write to the database. Is there any way I can achieve this? ---
**Author:** ![ced](https://discuss-cdn.tryton.org/user_avatar/discuss.tryton.org/ced/32/1237_2.png) [@ced](https://discuss.tryton.org/u/ced)\ **Post date:** [February 4, 2025, 2:41pm UTC](https://discuss.tryton.org/t/fault-cannot-execute-nextval-in-a-read-only-transaction/8275/2 "2025-02-04T14:41:42Z")
`on_change` methods are execute in read-only transaction because we decided that it was a bad idea to allow them to have side-effect (because there are no guarantee that the client will call them neither in which order). So instead of creating parameters, you should assign new parameter instances (unsaved) to the `One2Many`. Also you must not use a `on_change_with` because this form can only return a value for the corresponding field but instead probably `on_change_analysis_type`. ---
**Author:** ![maehder](https://discuss-cdn.tryton.org/letter_avatar_proxy/v4/letter/m/cdc98d/32.png) [@maehder](https://discuss.tryton.org/u/maehder)\ **Post date:** [February 4, 2025, 3:16pm UTC](https://discuss.tryton.org/t/fault-cannot-execute-nextval-in-a-read-only-transaction/8275/3 "2025-02-04T15:16:56Z")
Thank you a lot! The following code works like a charm: ```auto @fields.depends('analysis_template', 'parameter') def on_change_analysis_template(self): if self.analysis_template: self.parameter = self.analysis_template.parameter ``` However, when I create an ‘analysis’ record from another form it gives me the following error “Fault: (Pool().get(‘toxicology.analysis’)(\*\*{}), ‘parameter’)”. I guess it’s due to the fact, that the record wasn’t yet created. Would it make sense to implement an “On Creation”-Trigger for that case? ---
**Author:** ![ced](https://discuss-cdn.tryton.org/user_avatar/discuss.tryton.org/ced/32/1237_2.png) [@ced](https://discuss.tryton.org/u/ced)\ **Post date:** [February 4, 2025, 3:57pm UTC](https://discuss.tryton.org/t/fault-cannot-execute-nextval-in-a-read-only-transaction/8275/4 "2025-02-04T15:57:03Z")
I do not know if `parameter` is a `One2Many` or a `Many2Many` but if it is a `One2Many` you can not assign existing instance to it but you must assign new unsaved instances. ---
**Author:** ![maehder](https://discuss-cdn.tryton.org/letter_avatar_proxy/v4/letter/m/cdc98d/32.png) [@maehder](https://discuss.tryton.org/u/maehder)\ **Post date:** [February 4, 2025, 4:01pm UTC](https://discuss.tryton.org/t/fault-cannot-execute-nextval-in-a-read-only-transaction/8275/5 "2025-02-04T16:01:05Z")
It is a One2Many. How do I create an unsaved instance? I’m a bit puzzled. Maybe you already have a tryton example. That would suffice. ---
**Author:** ![ced](https://discuss-cdn.tryton.org/user_avatar/discuss.tryton.org/ced/32/1237_2.png) [@ced](https://discuss.tryton.org/u/ced)\ **Post date:** [February 4, 2025, 4:11pm UTC](https://discuss.tryton.org/t/fault-cannot-execute-nextval-in-a-read-only-transaction/8275/6 "2025-02-04T16:11:11Z")
You just need to instantiate the class. From your example it could be something like: ```auto @fields.depends('analysis_template', 'parameter') def on_change_analysis_template(self): pool = Pool() Parameter = pool.get('toxicology.parameter') if self.analysis_template: parameter = [] for param in self.analysis_template.parameter: parameter.append(Parameter( 'analyte': param.analyte, 'result': param.result, 'uom': param.uom)) self.parameter = parameter ``` _PS: I would strongly suggest to use plural form for `xxx2Many` fields, the code would be much more easier to read and reason._ ---
**Author:** ![maehder](https://discuss-cdn.tryton.org/letter_avatar_proxy/v4/letter/m/cdc98d/32.png) [@maehder](https://discuss.tryton.org/u/maehder)\ **Post date:** [February 5, 2025, 1:48pm UTC](https://discuss.tryton.org/t/fault-cannot-execute-nextval-in-a-read-only-transaction/8275/7 "2025-02-05T13:48:32Z")
Thank you for your code suggestion, instantiating the class makes of course sense. I adjusted the syntax of the instantiating and included your suggestion of making xxx2Many field names plural. I ended up with this method: ```auto @fields.depends('analysis_template', 'parameters') def on_change_analysis_template(self): pool = Pool() Parameter = pool.get('toxicology.parameter') if self.analysis_template: parameters = [] for param in self.analysis_template.parameters: parameters.append(Parameter( analysis = self.id, analyte = param.analyte, result = param.result, uom = param.uom and param.uom.id, )) self.parameters = parameters ``` Again it works within a created record, it works when creating a new “Analysis” record from the “Analysis” Tree View, but not if I try to create a new “Analysis” record from within another form (e. g. a “Sample” form that is linked to “Analysis” using another One2Many. Then it gives me this error: ```auto Traceback (most recent call last): File "/trytond/wsgi.py", line 110, in dispatch_request return endpoint(request, **request.view_args) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ File "/trytond/protocols/dispatcher.py", line 44, in rpc return methods.get(request.rpc_method, _dispatch)( ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ File "/trytond/wsgi.py", line 76, in wrapper return func(request, *args, **kwargs) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ File "/trytond/protocols/wrappers.py", line 208, in wrapper result = func(request, pool, *args, **kwargs) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ File "/trytond/protocols/dispatcher.py", line 221, in _dispatch result = rpc.result(meth(inst, *c_args, **c_kwargs)) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ File "/trytond/model/fields/field.py", line 195, in on_change_result return record._changed_values ^^^^^^^^^^^^^^^^^^^^^^ File "/trytond/model/modelview.py", line 891, in _changed_values assert hasattr(init_record, fname), (init_record, fname) ^^^^^^^^^^^^^^^^^^^^^^^^^^^ AssertionError: (Pool().get('toxicology.analysis')(**{}), 'parameters') Fault: (Pool().get('toxicology.analysis')(**{}), 'parameters') ``` I don’t understand why one unsaved record works and the other one does not. I hope you have any idea. ---
**Author:** ![ced](https://discuss-cdn.tryton.org/user_avatar/discuss.tryton.org/ced/32/1237_2.png) [@ced](https://discuss.tryton.org/u/ced)\ **Post date:** [February 5, 2025, 2:06pm UTC](https://discuss.tryton.org/t/fault-cannot-execute-nextval-in-a-read-only-transaction/8275/8 "2025-02-05T14:06:14Z")
> [@maehder](#): > > ` analysis = self.id,` You must not set the reverse `Many2One` value especially when it is a negative id. ---
**Author:** ![maehder](https://discuss-cdn.tryton.org/letter_avatar_proxy/v4/letter/m/cdc98d/32.png) [@maehder](https://discuss.tryton.org/u/maehder)\ **Post date:** [February 5, 2025, 2:43pm UTC](https://discuss.tryton.org/t/fault-cannot-execute-nextval-in-a-read-only-transaction/8275/9 "2025-02-05T14:43:27Z")
Thank you for your answer. I deleted the line. But the error persists. ---
**Author:** ![ced](https://discuss-cdn.tryton.org/user_avatar/discuss.tryton.org/ced/32/1237_2.png) [@ced](https://discuss.tryton.org/u/ced)\ **Post date:** [February 5, 2025, 3:06pm UTC](https://discuss.tryton.org/t/fault-cannot-execute-nextval-in-a-read-only-transaction/8275/10 "2025-02-05T15:06:56Z")
It looks like the client does not have the `parameters` field in the view. So it is not possible to calculate accurate modification. ---
**Author:** ![maehder](https://discuss-cdn.tryton.org/letter_avatar_proxy/v4/letter/m/cdc98d/32.png) [@maehder](https://discuss.tryton.org/u/maehder)\ **Post date:** [February 5, 2025, 3:27pm UTC](https://discuss.tryton.org/t/fault-cannot-execute-nextval-in-a-read-only-transaction/8275/11 "2025-02-05T15:27:39Z")
That was the last piece of the puzzle. Thank you very much for your patience! ---
**Author:** ![system](https://discuss-cdn.tryton.org/uploads/default/original/1X/c6f8ec0a40525cdcd50058c734283450a4b3d38b.png) [@system](https://discuss.tryton.org/u/system)\ **Post date:** [February 6, 2025, 3:28pm UTC](https://discuss.tryton.org/t/fault-cannot-execute-nextval-in-a-read-only-transaction/8275/12 "2025-02-06T15:28:37Z")
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