一个模型如何具有多个具有与值相同类型的模型的键？

Question

I'm making an application and I should make a Model which have 2 keys that saves models of the same type.

It's not easy to express in English so I will upload the image of the situation.

In Food Pair Model ( or we can call it table I think )

I want to refer Food Model But I wasn't able to use ForeignKey or ManyToManyField .

ERRORS:
food_test.FoodQuestion.left_food: (fields.E304) Reverse accessor for 'FoodQuestion.left_food' clashes with reverse accessor for 'FoodQuestion.right_food'.
    HINT: Add or change a related_name argument to the definition for 'FoodQuestion.left_food' or 'FoodQuestion.right_food'.
food_test.FoodQuestion.right_food: (fields.E304) Reverse accessor for 'FoodQuestion.right_food' clashes with reverse accessor for 'FoodQuestion.left_food'.
    HINT: Add or change a related_name argument to the definition for 'FoodQuestion.right_food' or 'FoodQuestion.left_food'.

I don't know what database relation to use in this case and how to make it.

What can I use for this case?

Answer 1

You must define a unique related_name for each ForeignKey field in your FoodPair model.

class FoodPair(models.Model):
    first_food = models.ForeignKey(Food, related_name="first_food")
    second_food = models.ForeignKey(Food, related_name="second_food")
    what_i_buy = models.ForeignKey(Food, related_name="what_i_buy")

If related_name is not defined Django automatically sets it and when there is multiple ForeignKey fields pointing to the same model the names clash.

Answer 2

When you create a ForeignKey from one model to another Django will dynamically create a property on the model being referenced that will return a QuerySet with all objects that have the foreign key to that object

For example

class Foo(models.Model):
    pass

class Bar(models.Model):
    foo = models.ForeignKey(Foo, on_delete=models.CASCADE)

foo = Foo.objects.create()
bar = Bar.objects.create(foo=foo)
foo.bar_set.all()  # This will return a queryset containing foo

By default this property will be <model_name_lowercase>_set . In your case because you have 2 foreign keys from one model to the same model Django is trying to create the same property on the Food model for each foreign key.

To get around this issue you can specify the name of this property using related_name , if you set this to '+' no reverse relation will be made at all or give them unique names

class FoodQuestion(models.Model):
    left_food = models.ForeignKey(Food, on_delete=models.CASCADE, related_name='+')
    left_food = models.ForeignKey(Food, on_delete=models.CASCADE, related_name='+')