如何检查值是否出现在Django模型列表的字段中？

Question

我有一个 Django 模型列表，我想检查字符串值是否出现在列表中任何模型的特定字段中的某处。请看下面的例子：

class something:
    animals = [] #type: list[Animals]

   def does_atleast_one_animal_make_that_sound(animal_sound):
       if animal_sound in ...# for each animal in animals, turn animal.sound into a list
            print('One of the animals in the list makes this sound')
       else:
            print('No animal makes this sound')

如何正确编写带有“...”的函数部分？

Answer 1

如果模型实例存在于数据库中，您可以使用任何列出的方法（第一种和第二种基于来自this answer 的想法）：

1) 查询集的values_list:

class something:
    animals = [] #type: list[Animals]
    animals = [animal.id for animal in animals]

    def does_atleast_one_animal_make_that_sound(animal_sound):
       if animal_sound in Animal.objects.filter(id__in=animals).values_list('sound', flat=True):
            print('One of the animals in the list makes this sound')
       else:
            print('No animal makes this sound')  

2) 如果你使用exists，那就更好了：

class something:
    animals = [] #type: list[Animals]
    animals = [animal.id for animal in animals]

    def does_atleast_one_animal_make_that_sound(animal_sound):
       if Animal.objects.filter(id__in=animals, sound=animal_sound).exists():
            print('One of the animals in the list makes this sound')
       else:
            print('No animal makes this sound')  

3) 如果模型只被构建（Animal(...)），但没有保存到数据库，那么你可以使用纯python过滤：

class something:
    animals = [] #type: list[Animals]

   def does_atleast_one_animal_make_that_sound(animal_sound):
       if any(filter(lambda animal: animal.sound == animal_sound, animals)):
            print('One of the animals in the list makes this sound')
       else:
            print('No animal makes this sound')  

注意：通常最好将模型实例过滤委托给数据库（谈论animals变量的来源）。在这种情况下，第 3 个选项似乎是最好的，因为它不需要查询数据库。