如何在Django中自动更改模型字段 [英] How to automatically change model fields in django

问题描述

我有一个模型，我想知道是否可以设置一个条件来触发模型字段中的更改。例如，我有一个模型

I have a model and I want to know if it is possible to set a condition that triggers a change in the model field. For example, I have a model

class BillboardTracker(models.Model):
    client_name = models.CharField(max_length=400)
    entry_date = models.DateTimeField(default=datetime.now)
    duration = models.PositiveIntegerField()
    expiry_date = models.DateField()
    is_expired = models.BooleanField(default=False)

我想知道模型中是否可以有一个函数当到期日期到期时，使is_expired等于True。我试过了

I want to know if it is possible to have a function in the model that makes is_expired equals to True when the expiry date is up. I tried this

def expire(self):
     if datetime.now == self.expiry_date:
        self.is_expired = True

，但它不起作用。

推荐答案

使用@属性

这里最简单的事情是根本没有过期字段！不需要您需要的是财产。

Use a @property

The simplest thing here is not to have an is expired field at all! It's not needed. What you need is a property.

class BillboardTracker(models.Model):
    client_name = models.CharField(max_length=400)
    entry_date = models.DateTimeField(default=datetime.now)
    duration = models.PositiveIntegerField()
    expiry_date = models.DateField()

    @property
    def is_expired(self):
        if datetime.now > self.expiry_date:
            return True
        return False

记住，您不如果数据库中的某个字段与另一个字段（通过简单的计算）相同，则该字段中没有数据库。这样会自动消除您不必将项目标记为已过期的烦恼。

Remember, you don't have a field in a database, if that field is the same as another field with a simple calculation. This automatically eliminates your head ache of having to flag items as expired.

如果您要查找对象是否已过期。

If you want to find out if an object has expired.

 if instance.is_expired == True:
    print 'yes, that ones gone'

过滤

如果要检索整套对象，已过期

Filtering

If you wanted to retrieve a whole set of objects that have expired

BillboardTracker.objects.filter(expiry_date__le=datetime.now())

这就是为什么我提到您不需要存储易于计算的字段的原因。

This is why I mentioned that you don't need to store a field that can be easily calculated.

在大多数RDBMS中，布尔字段（例如is_expired列）无法有效地建立索引。因此，这实际上意味着，只要您在expiry_date字段上创建索引，上述查询就会比对该布尔字段的查询要快。

In most RDBMS a boolean field (such as your is_expired column) cannot be indexed effectively. So that actually means the above query will be faster than a query on that boolean field provided you create an index on the expiry_date field.

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