Django抽象与非抽象模型继承

Question

Consider these two model classes Book and Article :

class Book(models.Model):
    title = models.CharField(max_length=100)


class Article(models.Model):
    title = models.CharField(max_length=100)

Imagine this scenario: a book can have many annotations but an annotation can refer to only one book. Same thing for the article. I have developed two solutions:

Non-abstract solution

class Annotation(models.Model):
    body = models.TextField()
    article = models.ForeignKey(Article, blank=True)
    book = models.ForeignKey(Book, blank=True)

    class Meta:
        default_related_name = 'annotations'

Pros

• Code is DRY
• One can access the annotations with book.annotations or article.annotations

Cons

• Waste database memory (one of the two fields article , book must be none)

Abstract classes solution

class Annotation(models.Model):
    body = models.TextField()

    class Meta:
        abstract = True


class BookAnnotation(Annotation):
    book = models.ForeignKey(Book, blank=True)

    class Meta:
        default_related_name = 'book_annotations'


class ArticleAnnotation(Annotation):
    article = models.ForeignKey(Article, blank=True)

    class Meta:
        default_related_name = 'article_annotations'

Pros

• Clear separation for book and article annotations
• No memory waste

Cons

• Does not quite match the concept of annotation (the object type should be the same)
• One can access the annotations with book.book_annotations or article.article_annotations (kind of ugly isn't it?). I can not use annotations as related name because Django prohibit that, it says reverse query name for BookAnnotation clashes with field name ArticleAnnotation .

Which is the best approach to deal with such situation?

Answer 1

You might consider generic relations here.

class Annotation(models.Model):
    body = models.TextField()
    content_type = models.ForeignKey(ContentType, on_delete=models.CASCADE)
    object_id = models.PositiveIntegerField()
    annotated_object = GenericForeignKey('content_type', 'object_id')

class Book(models.Model):
    ...
    annotations = GenericRelation('Annotation')

class Article(models.Model):
    ...
    annotations = GenericRelation('Annotation')

now you can set annotation.annotated_object to any book or article instance, and in reverse do obj.annotations from both book and article objects.