class A(models.Model):
def calculate(self):
# calculate some field based on all self.b_set instances
class B(models.Model):
a = models.ForeignKey(A)
I'd like saving an instance of B to cause a function to fire on A, but efficiently.
我想保存一个B的实例，使一个函数在a上启动，但是很有效。
A naive implementation would be to hook up to the post_save signal emitted by B instances then run b.a.calculate(), but it would call a.calculate() N times.
一个简单的实现是连接到B实例发出的post_save信号，然后运行b.a.a calculate()，但是它会调用a.a calculate() N次。
Does anybody have any suggestions for how to implement this more efficiently?
对于如何更有效地实现这个目标，有人有什么建议吗?
I'm hitting a roadblock because I'm not sure how I would determine when the last B instance was successfully updated after which I'd want the A instance run its potentially expensive calculate() function.
我遇到了一个障碍，因为我不知道如何确定最后一个B实例何时被成功更新，然后我希望a实例运行它可能昂贵的compute()函数。
My best idea so far might be a "changed" flag on A and have a cron job pick up which A models need to be updated, but I wish it could be more accurate.
到目前为止，我最好的想法可能是在a上有一个“改变”的标志，并且有一个cron作业，这是一个需要更新的模型，但是我希望它能更准确。
1 个解决方案
#1
2
If you have a formula x = a+b+c then you would always want to recalculate x after any of the arguments a,b or c changes. In your model above, if the calculated field value in model A depends on the relationships in model B then you would always want to recalculate after one of B instances change. However that does not necessary means the post_save will run N times. It will only run once every time one of B instances get saved.
如果你有一个公式x = a+b+c那么你总是希望在参数a b或c发生变化后重新计算x。在上面的模型中，如果模型A中计算的字段值取决于模型B中的关系，那么您总是希望在其中一个实例发生更改后重新计算。然而，这并不意味着post_save将运行N次。它只在每次保存一个B实例时运行一次。
If B has many fields that don not affect the expensive A.calculate() function, then adding a temporary checksum around the B's fields that matter to calculate and store the value as a new B field. Your post_save then can recalculate the checksum to see whether A.calculate() needs to run again or not.
如果B有许多不影响昂贵的a .calculate()函数的字段，那么在B的字段周围添加一个临时校验和，这些字段很重要，可以将值作为新的B字段来计算和存储。然后，您的post_save可以重新计算校验和，看看是否需要再次运行A.calculate()。
#1
2
If you have a formula x = a+b+c then you would always want to recalculate x after any of the arguments a,b or c changes. In your model above, if the calculated field value in model A depends on the relationships in model B then you would always want to recalculate after one of B instances change. However that does not necessary means the post_save will run N times. It will only run once every time one of B instances get saved.
如果你有一个公式x = a+b+c那么你总是希望在参数a b或c发生变化后重新计算x。在上面的模型中，如果模型A中计算的字段值取决于模型B中的关系，那么您总是希望在其中一个实例发生更改后重新计算。然而，这并不意味着post_save将运行N次。它只在每次保存一个B实例时运行一次。
If B has many fields that don not affect the expensive A.calculate() function, then adding a temporary checksum around the B's fields that matter to calculate and store the value as a new B field. Your post_save then can recalculate the checksum to see whether A.calculate() needs to run again or not.
如果B有许多不影响昂贵的a .calculate()函数的字段，那么在B的字段周围添加一个临时校验和，这些字段很重要，可以将值作为新的B字段来计算和存储。然后，您的post_save可以重新计算校验和，看看是否需要再次运行A.calculate()。