在django中获取具有相同外键的所有记录

Question

我正在做我的第一个项目Django/Postgres，这是一个追踪植物育种项目的应用程序。我有一个Generation模型，它使用ForeignKey连接到Project模型(即，每个育种项目由多代后代组成)：

class Generation(models.Model):
   # ...
   project = models.ForeignKey(
     Project, 
     related_name="generations", 
     on_delete=models.CASCADE
     )

我想添加一个根据相关项目自动递增的generation_number字段-如果在生成表中已经有3条记录具有相同的外键，那么使用该FK创建的下一条记录应该被分配generation_number为4。我的理解是我不能使用AutoField，因为这不是主键，所以我正在尝试编写一个方法来计算具有相同FK的记录的数量并加1，类似于：

def increment_gen_number(self):
  last_count = Project.objects.filter(pk=self.project).count()
  return last_count+1

gen_number = models.IntegerField(default=increment_gen_number)

我猜有一些语法问题，因为我仍然是Python的新手，这感觉有点杂乱无章。我怎么才能让它工作呢？

Answer 1

您可以重写Generation模型上的save方法，也可以重写所需的逻辑。

把这个放到你的模型上。

def save(self, *args, **kwargs):
    # your logic
    super(GeeksModel, self).save(*args, **kwargs)

Answer 2

您可以使用信号使字段保持最新：

from django.db import models

class Project(models.Model):   
   n_generations = models.IntegerField(default=0)


class Generation(models.Model):
   project = models.ForeignKey(
     Project, 
     related_name="generations", 
     on_delete=models.CASCADE
     )

# Signals
from django.db.models.signals import post_save, post_delete
from django.dispatch import receiver
from django.db.models import F


@receiver(post_save, sender=Generation)
def add_project(sender, instance, created, **kwargs):
    if not created:
        return
    project = instance.project
    project.n_generations = F('n_generations') + 1
    project.save()
    project.refresh_from_db()

@receiver(post_delete, sender=Generation)
def rm_project(sender, instance, **kwargs):
    project = instance.project
    project.n_generations = F('n_generations') - 1
    project.save()
    project.refresh_from_db()

测试通过：

from django.test import TestCase
from ppapp.models import Project, Generation

class ProjectTestCase(TestCase):
    def setUp(self):
        Project.objects.create()

    def test_animals_can_speak(self):
        """Animals that can speak are correctly identified"""
        p1 = Project.objects.all()[0]

        g1 = Generation(project=p1) # +1
        g1.save()
        g2 = Generation(project=p1) # +1
        g2.save()
        g3 = Generation(project=p1) # +1
        g3.save()
        g4 = Generation(project=p1) # +1
        g4.save()
        p1.generations.create() # +1
        g6 = Generation(project=p1) # +1
        g6.save()
        g6.delete() # -1
        
        self.assertEqual(p1.n_generations, 5)

(venv) dani@localhost ppp $ ./manage.py test
Creating test database for alias 'default'...
System check identified no issues (0 silenced).
.
----------------------------------------------------------------------
Ran 1 test in 0.017s

OK
Destroying test database for alias 'default'...