flask-admin + wtforms: TypeError: populate_obj: не удается найти значение для заполнения из предоставленного объекта или входных данных / значений по умолчанию

Question

Я использую колбу-админ и wtforms. У меня есть db.Model и форма, которая включает в себя formField и некоторые дополнительные поля для db.Model:

db.Model:

class Usuarios(db.Model):
    __tablename__ = 'clienteacceso'
    id = db.Column(db.INTEGER, primary_key=True)
    id_cliente = db.Column(db.INTEGER, nullable=False)
    usuario_acceso = db.Column(db.String, nullable=False)
    activo = db.Column(db.INTEGER, nullable=False)

Это простая форма с кнопкой, которая нажимается для обновления полей формы в соответствии с usuario_acceso:

class AccesosForm(form.Form):
    boton = SubmitField(label='editar datos de acceso'

Форма (с некоторыми дополнительными полями для db.model)

class UsuariosForm(form.Form):
    id_cliente = fields.SelectField('CUIT cliente', coerce=int,
                                choices=query_cliente_id_cuit()
    )
    usuario_acceso = fields.StringField('Usuario')
    activo = fields.SelectField('Estado',
                            coerce=int,
                            choices=[
                                (2, 'Activo'),
                                (1, 'Suspendido temporalmente'),
                                (0, 'Baja del servicio')
                            ]
    )
    acceso = fields.FormField(AccesosForm) #this is a button to update fields according to "usuario_acceso" data
    # extra fields
    passwd = fields.StringField('Contraseña')
    plan =  fields.SelectField(
        'Plan',
        choices=query_radgroupreply_groupname()
    )
    ipv4 = fields.StringField('IPv4 fija (opc)')
    ipv6_link = fields.StringField('IPv6 link (recomendada)')
    ipv6_delegado = fields.StringField('IPv6 delegada (opc)')

    def validate(form):
        if request.method == 'POST':
            usuario = form.usuario_acceso.data
            if form.acceso.boton.data: #this is True if "acceso" button is clicked
                """
                update form fields data according to usuario_acceso
                """
            else:
                """
                commit form data
                """
class UsuariosAdmin(sqla.ModelView):
    column_labels = dict(id_cliente = 'Cliente',
                     apellido = 'Apellido',
                     usuario_acceso = 'Usuario',
                     activo = 'Estado'
    )

    column_formatters = dict(
        id_cliente = lambda v, c, m, p: id_cliente_to_nombre(id = m.id_cliente),
        activo = lambda v, c, m, p: estado_to_str(int( str(m.activo).split("'")[1]))
    )

    column_list = ['id_cliente', 'usuario_acceso', 'activo']
    column_sortable_list = column_list
    column_searchable_list = column_list
    column_filters = column_list

    form = UsuariosForm

Все задачи в функции «проверить» работают хорошо, но при нажатии кнопки «Сохранить» представление не отображается, так как эта ошибка:

builtins.TypeError

TypeError: populate_obj: cannot find a value to populate from the provided obj or input data/defaults

Traceback:

     File "/root/miniconda3/envs/flask2/lib/python3.6/site-packages/flask/app.py", line 2309, in __call__
    return self.wsgi_app(environ, start_response)
  File "/root/miniconda3/envs/flask2/lib/python3.6/site-packages/flask/app.py", line 2295, in wsgi_app
    response = self.handle_exception(e)
  File "/root/miniconda3/envs/flask2/lib/python3.6/site-packages/flask/app.py", line 1741, in handle_exception
    reraise(exc_type, exc_value, tb)
  File "/root/miniconda3/envs/flask2/lib/python3.6/site-packages/flask/_compat.py", line 35, in reraise
    raise value
  File "/root/miniconda3/envs/flask2/lib/python3.6/site-packages/flask/app.py", line 2292, in wsgi_app
    response = self.full_dispatch_request()
  File "/root/miniconda3/envs/flask2/lib/python3.6/site-packages/flask/app.py", line 1815, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/root/miniconda3/envs/flask2/lib/python3.6/site-packages/flask/app.py", line 1718, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "/root/miniconda3/envs/flask2/lib/python3.6/site-packages/flask/_compat.py", line 35, in reraise
    raise value
  File "/root/miniconda3/envs/flask2/lib/python3.6/site-packages/flask/app.py", line 1813, in full_dispatch_request
    rv = self.dispatch_request()
  File "/root/miniconda3/envs/flask2/lib/python3.6/site-packages/flask/app.py", line 1799, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/root/miniconda3/envs/flask2/lib/python3.6/site-packages/flask_admin/base.py", line 69, in inner
    return self._run_view(f, *args, **kwargs)
  File "/root/miniconda3/envs/flask2/lib/python3.6/site-packages/flask_admin/base.py", line 368, in _run_view
    return fn(self, *args, **kwargs)
  File "/root/miniconda3/envs/flask2/lib/python3.6/site-packages/flask_admin/model/base.py", line 2130, in edit_view
    if self.update_model(form, model):
  File "/root/miniconda3/envs/flask2/lib/python3.6/site-packages/flask_admin/contrib/sqla/view.py", line 1146, in update_model
    if not self.handle_view_exception(ex):
  File "/root/miniconda3/envs/flask2/lib/python3.6/site-packages/flask_admin/contrib/sqla/view.py", line 1103, in handle_view_exception
    return super(ModelView, self).handle_view_exception(exc)
  File "/root/miniconda3/envs/flask2/lib/python3.6/site-packages/flask_admin/contrib/sqla/view.py", line 1142, in update_model
    form.populate_obj(model)
  File "/root/miniconda3/envs/flask2/lib/python3.6/site-packages/wtforms/form.py", line 96, in populate_obj
    field.populate_obj(obj, name)
  File "/root/miniconda3/envs/flask2/lib/python3.6/site-packages/wtforms/fields/core.py", line 829, in populate_obj
    raise TypeError('populate_obj: cannot find a value to populate from the provided obj or input data/defaults')