Hi,

To solve the above problem you can try like this,

slip_id = inv_obj.create({
            'partner_id': self.partner_id.name,
            'order_date': self.date_order,
            'expiration_date': self.validity_date,
            'so_no': self.name,   
        })

After creating the record you can add the values to the one2many lines,

inv_line_list =[]
for orders in self.order_line:
    inv_line_data = {
        'product_id': orders.product_id.name,
        'connecting_field' : slip_id,
        'quantity': orders.product_uom_qty,
    }
    inv_line_list.append(inv_line_data)

inv_line_obj = self.env['model_name_of_line']
for data_record in inv_line_list:
    print inv_line_obj.create(data_record)

Thanks

is this a working suggestion, please let us know.
Thanks

This  is  my code after your suggestion 

class sale_func(models.Model):
    _inherit = 'sale.order'


# data mapping ###
@api.multi
def _create_slip(reco):
    inv_obj = reco.env['loading.slip']
    reco.ensure_one()
        slip_id = inv_obj.create({
        'partner_id': reco.partner_id.name,
        'order_date': reco.date_order,
        'expiration_date': reco.validity_date,
        'so_no': reco.name,
    })
    inv_line_list = []
    for orders in reco.order_line:
        inv_line_data = {
             'product_id': orders.product_id.name,
            'connecting_field': slip_id,
             'quantity': orders.product_uom_qty,
        }
    inv_line_list.append(inv_line_data)

    inv_line_obj = reco.env['loading.product']
    for data_record in inv_line_list:
        return inv_line_obj.create(data_record)
ORM of loading slip module:
class LoadingSlip(models.Model):
    _name = 'loading.slip'
    _description = 'loading information'
    invoice_line_ids = fields.One2many('loading.product', 'bh_id', 'Loading ID')
    partner_id = fields.Char("गार्हक",store=True)
    order_date = fields.Date("मिति")
    expiration_date = fields.Date("रुजु मिति")
   truck_no=fields.Char("गाडी नं.")
    so_no=fields.Char(string="सि.नं.",store=True)

    # many to one relaiton with loading.slip ###

class product_tree(models.Model):
    _name ='loading.product'
    _description = 'product tree'
    bh_id = fields.Many2one('loading.slip','Loading ID')
    sno = fields.Char("S.No")
    product_id = fields.Char("Product Name",store=True)
   quantity=fields.Char("Quantity",store=True)