Django Model Forward Declaration

machine_model.py

class Quotations(models.Model):
    quoted_date = models.DateField(verbose_name="Quoted Date")
    price = models.FloatField(verbose_name="Quoted Price")
    machine = models.ForeignKey('Machine', verbose_name = "Associated Machine")
    def __unicode__(self):
        return u"%s %s" %(self.machine.name,self.quoted_date,self.price)


class Machine(models.Model):
    name = models.CharField(max_length = 30)
    manufacturer = models.CharField(max_length = 30)
    manufactued_date = models.DateTimeField(verbose_name="Date Manufactuered")
    category = models.ForeignKey(MachineType,verbose_name = "category of machine")
    quotation = models.ManyToManyField(Quotations,verbose_name="Quotations History")
    def __unicode__(self):
        return self.name