What are Django model relationships in depth?

Django ORM supports three types of model relationships: ForeignKey (Many-to-One): class Article(models.Model): author = models.ForeignKey(User, on_delete=models.CASCADE, related_name="articles") category = models.ForeignKey(Category, on_delete=models.SET_NULL, null=True). on_delete options: CASCADE (delete children), PROTECT (prevent deletion if children exist), SET_NULL (set FK to null), SET_DEFAULT, DO_NOTHING. Access: article.author (forward), user.articles.all() (reverse via related_name). ManyToManyField: class Article(models.Model): tags = models.ManyToManyField(Tag, blank=True, through="ArticleTag"). Through model (extra fields on the relationship): class ArticleTag(models.Model): article = models.ForeignKey(Article, on_delete=models.CASCADE) tag = models.ForeignKey(Tag, on_delete=models.CASCADE) created_by = models.ForeignKey(User, on_delete=models.CASCADE) added_at = models.DateTimeField(auto_now_add=True). Access: article.tags.add(tag); article.tags.remove(tag); article.tags.set([t1, t2]); article.tags.all(); tag.article_set.all(). OneToOneField: class Profile(models.Model): user = models.OneToOneField(User, on_delete=models.CASCADE, related_name="profile"). Like ForeignKey but enforces uniqueness — only one Profile per User. Access: user.profile (direct, no _set). Self-referential: parent = models.ForeignKey("self", on_delete=models.CASCADE, null=True, blank=True, related_name="children"). Generic relations: use ContentType for polymorphic FK — point to any model. from django.contrib.contenttypes.fields import GenericForeignKey.