I have two model:

class 1st(models.Model):
    _name = '1stmodel'

    scode =  fields.Char('Code', size=3, required=True)    
    writable_field = fields.Char('copia_campo')

and:

class 2nd(models.Model):
    
    _name = '2ndmodel'
    _inherits = {'1stmodel': 'id_1stmodel'}

    id_1stmodel = fields.Many2one('1stmodel', 'Code', widget="many2one_list")
    client =  fields.Char(string='Client', size=100, required=True)

    @api.one
    def write_method(self):
        modify_field = self.env['1stmodel'].search([('id','=',self.id_1stmodel)])
        modify_field.write({'writable_field': "ok"})

I have put a button in the second view to call the write method:

<record id="view_2nd_form" model="ir.ui.view">
	<field name="name">2nd.form.view</field>
	<field name="model">2ndmodel</field>
	<field name="arch" type="xml">
		<form string="Bridge" duplicate="false">
		        <header>
		        	<group>
		        		<button name="act_finish" type="object" string="modify 1stmodel"/>
				</group>
		        </header>
			<group>
		        	<field name="id_1stmodel"/>
				<field name="client"/>
			</group>
		</form>
	</field>
</record>

When I push on the button, I get this error:

Odoo Server Error

Traceback (most recent call last):
  File "/home/odoo/http.py", line 530, in _handle_exception
    return super(JsonRequest, self)._handle_exception(exception)
  File "/home/odoo/http.py", line 567, in dispatch
    result = self._call_function(**self.params)
  File "/home/odoo/http.py", line 303, in _call_function
    return checked_call(self.db, *args, **kwargs)
  File "/home/odoo/service/model.py", line 113, in wrapper
    return f(dbname, *args, **kwargs)
  File "/home/odoo/http.py", line 300, in checked_call
    return self.endpoint(*a, **kw)
  File "/home/odoo/http.py", line 796, in __call__
    return self.method(*args, **kw)
  File "/home/odoo/http.py", line 396, in response_wrap
    response = f(*args, **kw)
  File "/home/odoo/addons/web/controllers/main.py", line 953, in call_button
    action = self._call_kw(model, method, args, {})
  File "/home/odoo/addons/web/controllers/main.py", line 941, in _call_kw
    return getattr(request.registry.get(model), method)(request.cr, request.uid, *args, **kwargs)
  File "/home/odoo/api.py", line 241, in wrapper
    return old_api(self, *args, **kwargs)
  File "/home/odoo/addons/mymodule/inserimento.py", line 315, in act_finish
    assignment_ids = assignment_obj.search(cr, uid, [('id', '=', record.id_tbl_contratto)], context=context)
  File "/home/odoo/api.py", line 241, in wrapper
    return old_api(self, *args, **kwargs)
  File "/home/odoo/models.py", line 1637, in search
    return self._search(cr, user, args, offset=offset, limit=limit, order=order, context=context, count=count)
  File "/home/odoo/api.py", line 241, in wrapper
    return old_api(self, *args, **kwargs)
  File "/home/odoo/models.py", line 4587, in _search
    query = self._where_calc(cr, user, args, context=context)
  File "/home/odoo/api.py", line 241, in wrapper
    return old_api(self, *args, **kwargs)
  File "/home/odoo/models.py", line 4412, in _where_calc
    where_clause, where_params = e.to_sql()
  File "/home/odoo/osv/expression.py", line 1245, in to_sql
    q, p = self.__leaf_to_sql(leaf)
  File "/home/odoo/osv/expression.py", line 1112, in __leaf_to_sql
    "Invalid value %r in domain term %r" % (right, leaf)
AssertionError: Invalid value 1stmodel(6,) in domain term ('id', '=', 1stmodel(6,))

Could anyone tell me where is my error, please?

Thanks !