Django | 反向关系添加对象：多对一

在关联的模型中，比如多对一或者多对多这些类型，在某些场景下，比如简单快速为特定用户添加标签，为了快速添加对象，会使用反向添加，这里简单描述多对一场景下

django 1.10.3

1 非自定义模型主键 1.1 模型示例 @python_2_unicode_compatible class Author(models.Model): name = models.CharField(max_length=30) def __str__(self): return self.name @python_2_unicode_compatible class Posts(models.Model): headline = models.CharField(max_length=100) author = models.ForeignKey(Author, related_name="author_posts") def __str__(self): return self.headline 1.2 操作流程 In [6]: author2 = Author.objects.create(name='paul') In [7]: post2 = Posts(headline='hello post2') In [11]: author2.author_posts.add(post2, bulk=False) BEGIN Execution time: 0.000018s [Database: default] INSERT INTO "apple_posts" ("headline", "author_id") VALUES ('hello post2', 2) Execution time: 0.000314s [Database: default] 重复添加同一实例 In [12]: author2.author_posts.add(post2, bulk=False) BEGIN Execution time: 0.000018s [Database: default] UPDATE "apple_posts" SET "headline" = 'hello post2', "author_id" = 2 WHERE "apple_posts"."id" = 3 Execution time: 0.000257s [Database: default]

这里添加要注意 bulk=False

2 自定义模型主键 2.1 模型示例 import uuid class BaseBackBone(models.Model): id = models.UUIDField(primary_key=True, default=uuid.uuid1().hex, editable=False, max_length=32) class Meta: abstract = True @python_2_unicode_compatible class AuthorWithId(BaseBackBone): name = models.CharField(max_length=30) def __str__(self): return self.name @python_2_unicode_compatible class PostsWithId(BaseBackBone): headline = models.CharField(max_length=100) author = models.ForeignKey(AuthorWithId, related_name="author_posts") def __str__(self): return self.headline 2.2 操作流程 In [2]: author1 = AuthorWithId.objects.create(name='paul') In [3]: post1 = PostsWithId(headline='hello post1 with id') In [5]: author1.author_posts.add(post1, bulk=False) BEGIN Execution time: 0.000019s [Database: default] UPDATE "apple_postswithid" SET "headline" = 'hello post1 with id', "author_id" = '7d9d0f91ad6f11e6b0c1f45c89a84eed' WHERE "apple_postswithid"."id" = '7d9d0f91ad6f11e6b0c1f45c89a84eed' Execution time: 0.000141s [Database: default] INSERT INTO "apple_postswithid" ("id", "headline", "author_id") SELECT '7d9d0f91ad6f11e6b0c1f45c89a84eed', 'hello post1 with id', '7d9d0f91ad6f11e6b0c1f45c89a84eed' Execution time: 0.000291s [Database: default] 重复添加同一实例 In [6]: author1.author_posts.add(post1, bulk=False) BEGIN Execution time: 0.000021s [Database: default] UPDATE "apple_postswithid" SET "headline" = 'hello post1 with id', "author_id" = '7d9d0f91ad6f11e6b0c1f45c89a84eed' WHERE "apple_postswithid"."id" = '7d9d0f91ad6f11e6b0c1f45c89a84eed' Execution time: 0.001262s [Database: default] 3 总结

使用自定义主键添加对象时，数据库会多做一次查询操作，因为 Django 需要验证是更新还是插入操作。在使用自定义主键场景下，会增加数据库的查询操作。