Triggers on_write do not trigger for always True conditions
It is not possible to have a trigger which gets triggered at every write of the record.
This is because write method looks for trigger_eligibles [1] before beginning the write and trigger eligibles do not return records which already satisfy the trigger condition [2].
Finally trigger_write is only triggered on the records which returned negative on the eligible_check [3].
So for example: if the condition is True:
* The record is ignored at [1] because of [2]
[1] http://hg.tryton.org/2.6/trytond/file/271dd306761f/trytond/model/modelsql.py#l712
[2] http://hg.tryton.org/2.6/trytond/file/271dd306761f/trytond/model/modelstorage.py#l166
[3] http://hg.tryton.org/2.6/trytond/file/271dd306761f/trytond/model/modelsql.py#l950