model：

class ModelA(models.model):     
	​_name = 'model.a'
	​
   line_ids = fields.One2many('model.b', 'a_id', string="Detail")

class ModelB(models.model):     
	​_name = 'model.b'
	
   a_id = fields.many2one('model.a', string="Head")​
   line_ids = fields.One2many('model.c', 'b_id', string="Detail")

class ModelC(models.model):     
	​_name = 'model.c'
	
	​b_id = fields.many2one('model.b', string="Head")​​
   line_ids = fields.One2many('model.b', 'a_id', string="Detail")