class ArticleArticle(models.Model):

    _name = 'article.article'

    name = fields.Char(string='Lib :', required=True)

    price_unit = fields.Float(string='Prix :')

    q_int_en_stock = fields.Integer(string='Stock initiale :')

    img_article = fields.Binary("Image", attachment=True )

class IndexxLineline(models.Model):

 _name = 'indexx.lineline'

 name = fields.Char(string='Reference', index=True)

 releve_id = fields.Many2one('indexx.indexx', string='Releve')

 article_id = fields.Many2one('article.article', string='Article')

 p = fields.Float(string='Unit Price', readonly=True)

 index_ancien = fields.Float(string='Ancien Index')

 index_nouvel = fields.Float(string='Nouvel Index')

 price_subtotal = fields.Float(compute='_get_lecalcul')

when i select an Article  i need to get his price_unit and put it in the field p, how can i write the onchange function ?!