In odoo, I have 2 fields: state_id and district_id. Essentially, I want to select a state, then when selecting a district, it will only show the districts that belong to that state. I usually do it like this:
t will create a dummy field

domain_district = fields.Char(compute='_compute_domain_district')

and calculate for that field, this field will store the id of the district school that you need to display

 @api.depends('state_id')
    def _compute_domain_district(self):
        for rec in self:
            if rec.state_id:
                districts = self.env['res.country.district'].sudo().search([('state_id', '=', rec.state_id.id)])
                domain = [('id', 'in', districts.ids)] if districts else []
            else:
                domain = []
            rec.domain_district = json.dumps(domain)

inside xml t just need

<field name="district_id" invisible="country_code != 'VN'"  placeholder="District" domain="domain_district"
                               nolabel="1"/>

This method worked quite well, but I found it long and complicated because I had to create additional calculation fields, so I wanted a simpler approach than using the onchange function, I did the following:

 @api.onchange('state_id', 'district_id')
   def _onchange_state_id(self):
       result = {}
       if self.state_id:
           result['domain'] = {'district_id': [('state_id', '=', self.state_id.id)]}
       else:
           result['domain'] = {'district_id': [('id', 'in', [])]}
       return result

But this function doesn’t seem to be correct, even though I’ve selected state_id, when selecting district_id, it doesn’t filter out the correct records, it still shows all district_id, I need your help for the problem. This ?