class Content(ModelSQL, ModelView):
    __name__ = 'content'
    original_data = fields.Many2One(
        'ir.attachment', domain=[('resource', '=', ('content', Eval('id')))])
    original_data = fields.Many2One(
        'ir.attachment', domain=[('resource', '=', (__name__, Eval('id')))])

Both 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