I have two classes  purchase_requisition and purchase_requisition_line .I want to show a value from purchase _requisition_line  class in purchase_requisition class.So far i have done this much 

    class purchase_requisition_line(osv.osv):
        _name = "purchase.requisition.line"
        _description = "Purchase Requisition Line"
        _rec_name = 'product_id'
        _columns = {
            'product_id': fields.many2one('product.product','Product', domain=[('purchase_ok', '=', True)]),
            'requisition_id': fields.many2one('purchase.requisition', 'Requisition', ondelete='cascade'),
        }

and

class purchase_requisition(osv.osv):
    _name = "purchase.requisition"
    _description = "Purchase Requisition"
    _inherit = ['mail.thread', 'ir.needaction_mixin']
    _columns = {
        'line_ids': fields.one2many('purchase.requisition.line', 'requisition_id', 'Products to Purchase', states={'done': [('readonly', True)]}, copy=True),
        'product': fields.many2one('line_ids.product_id', string='Product', store=True),
    }

I want to show product_id (ie Product name) in treeview of class but failed to do so, is there something i am missing?