supprose i have this 2 class: 

class Personne(models.Model):

    _name = "personne"

name = fields.Char(string="name")

birth = fields.Date(string="Birth date")

class inscription(models.Model):

    _name = "inscription"

    name = fields.Char(string="Date")

personne = fields.Many2one('personne', string='Personne')

    birth = fields.Many2one('personne', string='Birth date')

in class "inscription" the value of birth should be related to shoosed personne

how can i set birth value automaticaly after personne choosed?