如何在Django中使用通用ForeignKey定义模式

How to define Mode with generic ForeignKey in Django

我对姜戈不熟悉

我想用以下逻辑创建模型：

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class ExerciseCardio(models.Model):
pass

class ExerciseWeights(models.Model):
pass

class Exercise(models.Model):
name = models.CharField(max_length=100, default='')

EXERCISE_TYPE_CHOICES = (
(1, 'cardio'),
(2, 'Weights'),
)

exercise_type = models.PositiveSmallIntegerField(
choices=EXERCISE_TYPE_CHOICES, default=2)

if exercise_type == 1:
exercise_model_type = models.ForeignKey(ExerciseCardio, on_delete=models.CASCADE, default=0)
elif exercise_type == 2:
exercise_model_type = models.ForeignKey(ExerciseWeights, on_delete=models.CASCADE, default=0)

def __str__(self):
return self.name

我知道它看起来很难看但一定有办法

是的，有一种方法：可以使用Djangos通用关系。

其要点如下：

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from django.contrib.contenttypes.fields import GenericForeignKey
from django.contrib.contenttypes.models import ContentType

class Exercise(models.Model):
EXERCISE_TYPE_CHOICES = (
(1, 'cardio'),
(2, 'Weights'),
)

name = models.CharField(
max_length=100, default='')
exercise_type = models.PositiveSmallIntegerField(
choices=EXERCISE_TYPE_CHOICES, default=2)
content_type = models.ForeignKey(ContentType, on_delete=models.CASCADE)
object_id = models.PositiveIntegerField()
content_object = GenericForeignKey('content_type', 'object_id')

在您看来，在创建Exercise实例时，您必须选择正确模型的ContentType，可能如下所示：

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obj = Exercise()
obj.exercise_type = ...
if obj.exercise_type == 1:
obj.content_type = ContentType.objects.get_for_model(ExerciseCardio)
else:
obj.content_type = ContentType.objects.get_for_model(ExerciseWeights)

正如你所指出的和拉尔夫所描述的，Django中的实际通用外键仍然笨重难看。

但是，您要讨论的是一些特定类型，它们需要以特定的方式进行操作，我认为这是一个很好的继承候选，可以借助于库中的自定义管理器：django-model-utils.managers.InheritanceManager。

models.py：

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from django.db import models
from model_utils.managers import InheritanceManager

class Exercise(models.Model):
name = models.CharField(max_length=32)

objects = InheritanceManager()

def __str__(self):
return"{n} ({t})".format(n=self.name, t=type(self))

class ExerciseCardio(Exercise):
pass

class ExerciseWeights(Exercise):
pass

示例(在Django Shell中，使用我的高级测试应用程序eh)：

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from eh.models import ExerciseCardio, Exercise, ExerciseWeights

c = ExerciseCardio.objects.create(name="Cardio!")
w = ExerciseWeights.objects.create(name="Weights!")

print(Exercise.objects.filter(name="Cardio!").select_subclasses().get())
# Cardio! (<class 'eh.models.ExerciseCardio'>)

for e in Exercise.objects.all().select_subclasses():
print(e)
# Cardio! (<class 'eh.models.ExerciseCardio'>)
# Weights! (<class 'eh.models.ExerciseWeights'>)