I have a model: MyModel whose inherit from 'account.analytic.line' and another model ResConfigSettings whose inherit from: 'res.config.settings'

In MyModel is first_field(many2one), In ResConfigSettings is second_field(Many2one)
And when wrtie:

@api.model
def default_get(self, fields):
res = super(MyModel, self).default_get(fields)
second_field = self.env['ir.config_parameter'].sudo().get_param('second_field')
if 'second' not in fields:
fields.append('second_field')
res['first'] = second_field.id
return res

I get a error:

res['first_field'] = second_field.id
Exception raise exception.with_traceback(None) from new_cause
AttributeError: 'str' object has no attribute 'id'

In addition, I wonder if it can be done using the method, since the fields are in other models.

raise exception.with_traceback(None) from new_cause
AttributeError: 'bool' object has no attribute 'id'

# -*- coding: utf-8 -*-
from odoo import models, fields, api

class ResConfigSettings(models.TransientModel):
_inherit = 'res.config.settings'
assign_tax = fields.Boolean('Assign Tax Group', default=False, store=True, readonly=False)
tax_group = fields.Many2one('account.account', 'Tax group', readonly=False, store=True)

(other methods)

class TimesheetAccountAssignment(models.Model):
_description = "Timesheet financial account assignment"
_inherit = "account.analytic.line"

general_account_id = fields.Many2one('account.account', string="Accounting account",
compute='_compute_display_name', readonly=False)

@api.model
def default_get(self, fields):
res = super(TimesheetAccountAssignment, self).default_get(fields)
tax_group = self.env['ir.config_parameter'].sudo().get_param('tax_group')
print(tax_group) # it's int (7)
field_id = self.env['account.analytic.line'].search([('id', '=', tax_group)]).id
print(field_id) # return value bool - False
if 'tax_group' not in fields:
fields.append('tax_group')
res['general_account_id'] = field_id.id
return res

At first check that record exist or not, later access by id
field_id = self.env['account.analytic.line'].search([('id', '=', tax_group)])
if field_id:
//followed by your code

Ok, problem was in field_id = self.env['account.analytic.line'].search([('id', '=', tax_group)]),
should be field_id = self.env['account.account'].search([('id', '=', tax_group)]).

Now I have a:
raise exception.with_traceback(None) from new_cause
ValueError: Invalid field 'tax_group' on model 'account.analytic.line'