如何正确地在Django中建模以下内容

Question

我有以下表格。 ComponentBuild表有两列'comp_pn'和'parent_pn'，它们都是ForeignKeys。 Django抱怨未知列'product_parent_pn.product_id_id 。建模这些关系的正确方法是什么？

class Product(models.Model):
    name = models.CharField(max_length=80)

    def __unicode__(self):
        return self.name

class ProductParentPn(models.Model):
    part_no = models.CharField(max_length=80)
    product_id = models.ForeignKey(Product)
    def __unicode__(self):
        return self.part_no

class CompMap(models.Model):
    component = models.CharField(max_length=50)
    part_no = models.CharField(max_length=50)

    def __unicode__(self):
        return self.part_no 

class CompProductMap(models.Model):
    comp_id = models.IntegerField()
    prod_id = models.IntegerField()
    class Meta:
        db_table = u'comp_product_map'

class ComponentBuild(models.Model):
    comp_sn = models.CharField(max_length=35)
    parent_sn = models.CharField(max_length=15)
    comp_pn = models.ForeignKey(CompMap)
    parent_pn = models.ForeignKey(ProductParentPn)
    date = models.DateField(default=datetime.datetime.today())

    def __unicode__(self):
        return self.comp_sn