如何安全地从 django models.Model 类继承？

Question

I have a following DB structure:

class Word(models.Model):
    original = models.CharField(max_length=40)
    translation = models.CharField(max_length=40)

class Verb(Word):
    group = models.IntegerField(default=1)

In my view, I need to create a Word object first, and after determination of its group (depending on Word.original ), create a Verb object, and save it.

What is the best way to inherit from the Word class and save the object as Verb ?

There are several solutions that I've tried:

1) Modification of the __init__ method in Verb :

class Verb(Word):
    group = models.IntegerField(default=1)

    def __init__(self, base_word):
        self.original = base_word.original
        self.translation = base_word.translation

This causes a lot of errors, since I'm overriding the django's built-in __init__ method.

2) Using super().__init__() :

class Verb(Word):
    group = models.IntegerField(default=1)

    def __init__(self, base_word):
        super().__init__()
        self.original = base_word.original
        self.translation = base_word.translation

Apparently, this works pretty well:

base_word = Word()
new_verb = Verb(base_word)
new_verb.save()

But there are two problems:

1. It causes an error when trying to see the objects in django admin page:

__init__() takes 2 positional arguments but 9 were given

2. This is still too much code, it doesn't feel right. I still need to write this:

self.original = base_word.original
self.translation = base_word.translation

in every subclass. And this is just an example. In real project, I have much more fields. I suppose there is a more elegant solution.

Answer 1

Overriding __init__ is not the right way to do this. Django models perform a lot of behind the scenes work, which overriding __init__ can conflict with, unless you do it in a safe way by following these rules:

• Don't alter the signature of __init__ -- meaning you shouldn't change the arguments that the method accepts.
• Perform the custom __init__ logic after calling the super().__init__(*args, **kwargs) method.

In this particular case, you might use django's proxy model inheritance features .

VERB = "V"
NOUN = "N"
# ...
WORD_TYPE_CHOICES = (
    (VERB, "Verb"),
    (NOUN, "Noun"),
    # ...
)

class Word(models.Model):
    original = models.CharField(max_length=40)
    translation = models.CharField(max_length=40)

    WORD_TYPE = ""  # This is overridden in subclasses

    word_type = models.CharField(
        max_length=1,
        blank=True,
        editable=False,  # So that the word type isn't editable through the admin.
        choices=WORD_TYPE_CHOICES,
        default=WORD_TYPE,  # Defaults to an empty string
    )

    def __init__(self, *args, **kwargs):
        # NOTE: I'm not 100% positive that this is required, but since we're not
        # altering the signature of the __init__ method, performing the
        # assignment of the word_type field is safe.
        super().__init__(*args, **kwargs)
        self.word_type = self.WORD_TYPE

    def __str__(self):
        return self.original

    def save(self, *args, **kwargs):
        # In the save method, we can force the subclasses to self-assign
        # their word types.
        if not self.word_type:
            self.word_type = self.WORD_TYPE
        super().save(*args, **kwargs)

class WordTypeManager(models.Manager):
    """ This manager class filters the model's queryset so that only the
    specific word_type is returned.
    """
    def __init__(self, word_type, *args, **kwargs):
        """ The manager is initialized with the `word_type` for the proxy model. """
        self._word_type = word_type
        super().__init__(*args, **kwargs)

    def get_queryset(self):
        return super().get_queryset().filter(word_type=self._word_type)

class Verb(Word):
    # Here we can force the word_type for this proxy model, and set the default
    # manager to filter for verbs only.
    WORD_TYPE = VERB
    objects = WordTypeManager(WORD_TYPE)

    class Meta:
        proxy = True

class Noun(Word):
    WORD_TYPE = NOUN
    objects = WordTypeManager(WORD_TYPE)

    class Meta:
        proxy = True

Now we can treat the different word types as if they were separate models, or access all of them together through the Word model.

>>> noun = Noun.objects.create(original="name", translation="nombre")
>>> verb = Verb(original="write", translation="escribir")
>>> verb.save()

# Select all Words regardless of their word_type
>>> Word.objects.values_list("word_type", "original")
<QuerySet [('N', 'name'), ('V', 'write')]>

# Select the word_type based on the model class used
>>> Noun.objects.all()
<QuerySet [<Noun: name>]>
>>> Verb.objects.all()
<QuerySet [<Verb: write>]>

This works with admin.ModelAdmin classes too.

@admin.register(Word)
class WordAdmin(admin.ModelAdmin):
    """ This will show all words, regardless of their `word_type`. """
    list_display = ["word_type", "original", "translation"]

@admin.register(Noun)
class NounAdmin(WordAdmin):
    """ This will only show `Noun` instances, and inherit any other config from
    WordAdmin.
    """