如何设置相互关联的Django模型字段

Question

I have the following simplified problem. Say that I have two Django models, namely a (P)roject and (H)ourly which stores hourly values for the project.

Now lets assume that there is a simple relation between these entities, eg: Hy = 2*Hx + P.par. Is there an elegant way to implement this? Where to put the help function:

def calc_y(x, par):
    return 2*x+par

class P(models.Model):
    par = FloatField()

class H(models.Model):
    p = models.ForeignKey(P, on_delete.CASCADE)
    x = FloatField()
    y = FloatField()

Answer 1

In case you do not need to query on y (only obtain the value), we can make it a property instead:

class P(models.Model):
    par = FloatField()

class H(models.Model):
    p = models.ForeignKey(P, on_delete.CASCADE)
    x = FloatField()

    

So we do not store it in the database: the property is evaluated when needed. So if you have a H instance h , then hy will let Python evaluate the value and return it.

The advantage of doing so is that we avoid data duplication, so it can not happen that the relation between the three variables is not satisfied, we also save on memory, since the database table will be smaller.

A problem with this approach is that since the value is not stored explicitly, querying on y is impossible (or at least not without some tricks). So we can not easily query the database for all H s where y < 0.5 for example.

Answer 2

You should put all the code with business logic in services.py of your Django app.

For example, your case should go like,

from .models import H, P

def calculate_y(x, par):
    obj = H.objects.get(x=x)  # if x is unique else use id
    obj.y = 2 * x + par
    obj.save(update_fields['y'])

You can call this function right after creating an object for H.