[tryton] Many2One domain for reference field in field definition

['Udo Spallek' via tryton]

Hi,
in a many2one field definition I want to set a domain for a
specific resource in ir.attachments. Is it possible to create such a
domain in a fields definition or do I need to use a function field?
My non working code looks like this::

class Content(ModelSQL, ModelView):
    __name__ = 'content'
    original_data = fields.Many2One(  # 1. try
        'ir.attachment', domain=[
            ('resource', '=', ('content', Eval('id')))])
    original_data = fields.Many2One(  # 2. try
        'ir.attachment', domain=[
            ('resource', '=', (__name__, Eval('id')))])
 
Both tries raises the following error:
 
Traceback (most recent call last):
  File "/usr/local/bin/trytond", line 6, in <module>
    exec(compile(open(__file__).read(), __file__, 'exec'))
  File "/ado/src/trytond/bin/trytond", line 80, in <module>
    trytond.server.TrytonServer(options).run()
  File "/ado/src/trytond/trytond/server.py", line 98, in run
    Pool(db_name).init(update=self.options.update, lang=lang)
  File "/ado/src/trytond/trytond/pool.py", line 140, in init
    self.start()
  File "/ado/src/trytond/trytond/pool.py", line 92, in start
    register_classes()
  File "/ado/src/trytond/trytond/modules/__init__.py", line 351, in
register_classes
    mod_file, pathname, description)
  File "/ado/src/collecting_society/__init__.py", line 5, in <module>
    from .collecting_society import *
  File "/ado/src/collecting_society/collecting_society.py", line 585,
in <module>
    class Content(ModelSQL, ModelView):
  File "/ado/src/collecting_society/collecting_society.py", line 614,
in Content
    'ir.attachment', domain=[('resource', '=', ('content',
Eval('id')))])
  File "/ado/src/trytond/trytond/model/fields/many2one.py", line 49, in
__init__
    depends=depends, context=context, loading=loading)
  File "/ado/src/trytond/trytond/model/fields/field.py", line 142, in
__init__
    assert string, 'a string is required'
AssertionError: a string is required

It seems I need to create a string for the resource, which is a fields
reference, but how can I do this with PYSON?

Do you have experience in this?

TIA, 
best Udo Spallek

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