Python 如何在 Django 中默认 IntegerField
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How to default IntegerField in Django
提问by BasiL
I was studying some Django, and came across a small problem.
我正在学习一些 Django，并遇到了一个小问题。
I am looking for a way to make suit_length equal to customer's height by default. This kind of code works for me:
我正在寻找一种默认情况下使suit_length 等于客户身高的方法。这种代码对我有用：
class Customer(models.Model):
name = models.CharField(max_length=20)
height = models.IntegerField(default=170)
class Suit(models.Model)
customer = models.ForeignKey(Customer)
suit_design = models.CharField(max_length=100)
suit_length = models.IntegerField(default=0)
def get_height(self):
self.suit_length = self.customer.height
return
But every time I create a new Suit its default suit_length = 0, and I have to run get_height() to get what I want. Is there a way to default suit_length to customer.height and to avoid running get_height() every time I create a new Suit? I am probably looking for smth like this:
但是每次我创建一个新的套装时，它的默认套装长度 = 0，我必须运行 get_height() 才能得到我想要的。有没有办法将suit_length默认为customer.height并避免每次创建新西装时都运行get_height()？我可能正在寻找这样的东西：
class Customer(models.Model):
name = models.CharField(max_length=20)
height = models.IntegerField(default=170)
class Suit(models.Model)
customer = models.ForeignKey(Customer)
suit_design = models.CharField(max_length=500)
suit_length = models.IntegerField(default=lambda:self.customer.height)
But this code doesn't work.
但是这段代码不起作用。
回答by paulus
Django uses the save() method to actually save the data. You may overwrite it like so:
Django 使用 save() 方法来实际保存数据。你可以像这样覆盖它：
class Suit(models.Model):
def save(self):
if self.suit_length == 0:
self.suit_length = self.customer.height
super(Suit, self).save()
It is not overwriting the default, but it achieves your goal.
它不会覆盖默认值，但可以实现您的目标。
回答by Bill Torcaso
You want an invariant to be true at run-time, which is that the initial length of a Suit is equal to the height of a Customer.
您希望不变量在运行时为真，即西装的初始长度等于客户的高度。
Doesn't the pythonic way to do this involve __ init__() ?
这样做的pythonic方法不涉及 __ init__() 吗？
class Suit(models.Model)
customer = models.ForeignKey(Customer)
suit_design = models.CharField(max_length=500)
suit_length = models.IntegerField(default=lambda:self.customer.height)
def __init__(self, args, kwargs):
super(models.Model, args, kwargs)
if kwargs['customer']:
self.customer = kwargs['customer']
# this creates the invariant
self.suit_length = self.customer.height
else:
raise ValueError("Must have a customer before constructing a Suit")
# More code for suit_design, etc.
}
