ValueError: Expected singleton: - Odoo v8

I have this method, which should loop on a One2many object, but the actual loop isn't working, I mean, if I add just one line it works fine, but if I add more than one line, than it throws me the singleton error:

@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'))

These are the two objects on which this method is based upon:

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

If I add more than one isbn on bsi.production.order.lines it throws me:

ValueError

Expected singleton: bsi.production.order.lines(10, 11)

Any ideas?

EDIT

The duplicate is a different situation, and actually I've changed my method to match the one explained in the other question, with no success. So it's not really, or at least not an api-only issue.

In your case, it's found more then one record set in order_lines and you tried to get isbn value from it.

Try with following code:

@api.multi
@api.depends('order_lines', 'order_lines.isbn')
def checkit(self):
    for record in self:
        if record.order_lines:
            for line in record.order_lines:
                if line.isbn:  
                    return line.isbn
        else:
            raise Warning(('Enter​ ​at least​ ​1​ ​ISBN to produce'))