Hi
Do you mean you would like to record the country_id that has been assigned to your model?
e.g. First when "Belgium" is selected, sequence_field will show "Belgium" and after "Canada" is select, sequence_field will show "Belgium Canada"?
If this is the case, you many try to make the sequence_field a compute field.
Example
Class TempTemp(models.Model)
_name = 'temp.temp'
country_id = fields.Many2one('res.country','Country')
sequence_field = fields.Char(compute='_get_sequence')
@api.depends('country_id')
def _get_sequence(self):
self.sequence_field += self.country_id.name