I have these models:

class bsi_production_order(models.Model):
    _name = 'bsi.production.order'

    name = fields.Char('Reference', required=True, index=True, copy=False, readonly='True', default='New')
    date = fields.Date(string="Production Date")
    production_type = fields.Selection([
        ('budgeted','Budgeted'),
        ('nonbudgeted','Non Budgeted'),
        ('direct','Direct Order'),
    ], string='Type of Order', index=True,  
    track_visibility='onchange', copy=False,
    help=" ")
    notes = fields.Text(string="Notes")
    order_lines = fields.One2many('bsi.production.order.lines', 'production_order', states={'finished': [('readonly', True)], 'cancel': [('readonly', True)]}, string="Order lines", copy=True)

class bsi_production_order_lines(models.Model):
    _name = 'bsi.production.order.lines'

    production_order = fields.Many2one('bsi.production.order', string="Production Orders")
    isbn = fields.Many2one('product.product', string="ISBN", domain="[('is_isbn', '=', True)]")
    qty = fields.Integer(string="Quantity")
    consumed_qty = fields.Float(string="Consumed quantity")
    remaining_qty = fields.Float(string="Remaining quantity", compute="_remaining_func")

    @api.onchange('qty', 'consumed_qty')
    def _remaining_func(self):
        if self.consumed_qty or self.qty:
            self.remaining_qty = self.consumed_qty - self.qty

I'm trying to loop into bsi.production.order.lines and check all the 'isbn' which are products, this is what I've tried so far:

1. @api.multi

       @api.depends('order_lines', 'order_lines.isbn')

       def checkit(self):

           for record in self:

               if self.order_lines.isbn:

                   return self.order_lines.isbn

               else:

                   raise Warning(('Enter​ ​at least​ ​1​ ​ISBN to produce'))

But If I add more than one product or 'isbn' to line, it throws me the 'Expected singleton' error.

How can I properly loop through bsi.production.order.lines?