在django中獲取具有相同外鍵的所有記錄

Question

我正在 Django/Postgres 中開展我的第一個項目，這是一個跟蹤植物育種項目的應用程序。 我有一個Generation模型，它連接到一個帶有ForeignKey的Project模型（即，每個育種項目由多代后代組成）：

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

我想添加一個generation_number字段，該字段根據關聯的項目自動遞增 - 如果 Generation 表中已經有 3 條具有相同外鍵的記錄，那么使用該 FK 創建的下一條記錄應該被分配一個generation_number為 4 . 我的理解是我不能使用AutoField因為這不是主鍵，所以我試圖編寫一個方法來計算具有相同 FK 的記錄數並加 1，例如：

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)

我猜有一些語法問題，因為我還是 Python 的新手，這感覺有點麻煩。 我怎樣才能讓它發揮作用？

Answer 1

您可以覆蓋 Generation 模型上的 save 方法，以及您想要的邏輯。

把這個放在你的模型上。

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

Answer 2

您可以使用信號使您的字段保持最新狀態：

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()

測試通過：

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'...