使用相关的模型聚合查询，而不仅仅是相关的ID值

Say I have three models:

假设我有三个模型:

from django.db import models    

class X(models.Model):
    y = models.ForeignKey(Y, on_delete=models.PROTECT)
    z = models.ForeignKey(Z, on_delete=models.PROTECT)
    a = models.DecimalField(...)
    b = models.DecimalField(...)
    ...some fields...

class Y(models.Model):
   ...some fields...

class Z(models.Model):
   ...some fields...
from dja