如何为具有多对多字段的 Django 模型创建对象? [英] How to create an object for a Django model with a many to many field?

问题描述

我的模型:

class Sample(models.Model):
    users = models.ManyToManyField(User)

我想在该模型中同时保存 user1 和 user2:

I want to save both user1 and user2 in that model:

user1 = User.objects.get(pk=1)
user2 = User.objects.get(pk=2)
sample_object = Sample(users=user1, users=user2)
sample_object.save()

我知道那是错的，但我相信你明白我想要做什么.你会怎么做?

I know that's wrong, but I'm sure you get what I want to do. How would you do it ?

推荐答案

您无法从未保存的对象创建 m2m 关系.如果你有 pks，试试这个:

You cannot create m2m relations from unsaved objects. If you have the pks, try this:

sample_object = Sample()
sample_object.save()
sample_object.users.add(1,2)

更新:阅读saverio 的回答，我决定更深入地调查这个问题.这是我的发现.

Update: After reading the saverio's answer, I decided to investigate the issue a bit more in depth. Here are my findings.

这是我最初的建议.它有效，但不是最佳的.(注意:我使用 Bars 和 Foo 而不是 Users 和 Sample，但是你明白了.

This was my original suggestion. It works, but isn't optimal. (Note: I'm using Bars and a Foo instead of Users and a Sample, but you get the idea).

bar1 = Bar.objects.get(pk=1)
bar2 = Bar.objects.get(pk=2)
foo = Foo()
foo.save()
foo.bars.add(bar1)
foo.bars.add(bar2)

它总共生成了 7 个查询:

It generates a whopping total of 7 queries:

SELECT "app_bar"."id", "app_bar"."name" FROM "app_bar" WHERE "app_bar"."id" = 1
SELECT "app_bar"."id", "app_bar"."name" FROM "app_bar" WHERE "app_bar"."id" = 2
INSERT INTO "app_foo" ("name") VALUES ()
SELECT "app_foo_bars"."bar_id" FROM "app_foo_bars" WHERE ("app_foo_bars"."foo_id" = 1  AND "app_foo_bars"."bar_id" IN (1))
INSERT INTO "app_foo_bars" ("foo_id", "bar_id") VALUES (1, 1)
SELECT "app_foo_bars"."bar_id" FROM "app_foo_bars" WHERE ("app_foo_bars"."foo_id" = 1  AND "app_foo_bars"."bar_id" IN (2))
INSERT INTO "app_foo_bars" ("foo_id", "bar_id") VALUES (1, 2)

我相信我们可以做得更好.您可以将多个对象传递给 add() 方法:

I'm sure we can do better. You can pass multiple objects to the add() method:

bar1 = Bar.objects.get(pk=1)
bar2 = Bar.objects.get(pk=2)
foo = Foo()
foo.save()
foo.bars.add(bar1, bar2)

正如我们所见，传递多个对象可以节省一个SELECT:

As we can see, passing multiple objects saves one SELECT:

SELECT "app_bar"."id", "app_bar"."name" FROM "app_bar" WHERE "app_bar"."id" = 1
SELECT "app_bar"."id", "app_bar"."name" FROM "app_bar" WHERE "app_bar"."id" = 2
INSERT INTO "app_foo" ("name") VALUES ()
SELECT "app_foo_bars"."bar_id" FROM "app_foo_bars" WHERE ("app_foo_bars"."foo_id" = 1  AND "app_foo_bars"."bar_id" IN (1, 2))
INSERT INTO "app_foo_bars" ("foo_id", "bar_id") VALUES (1, 1)
INSERT INTO "app_foo_bars" ("foo_id", "bar_id") VALUES (1, 2)

我不知道您还可以分配对象列表:

I wasn't aware that you can also assign a list of objects:

bar1 = Bar.objects.get(pk=1)
bar2 = Bar.objects.get(pk=2)
foo = Foo()
foo.save()
foo.bars = [bar1, bar2]

不幸的是，这会创建一个额外的 SELECT:

Unfortunately, that creates one additional SELECT:

SELECT "app_bar"."id", "app_bar"."name" FROM "app_bar" WHERE "app_bar"."id" = 1
SELECT "app_bar"."id", "app_bar"."name" FROM "app_bar" WHERE "app_bar"."id" = 2
INSERT INTO "app_foo" ("name") VALUES ()
SELECT "app_foo_bars"."id", "app_foo_bars"."foo_id", "app_foo_bars"."bar_id" FROM "app_foo_bars" WHERE "app_foo_bars"."foo_id" = 1
SELECT "app_foo_bars"."bar_id" FROM "app_foo_bars" WHERE ("app_foo_bars"."foo_id" = 1  AND "app_foo_bars"."bar_id" IN (1, 2))
INSERT INTO "app_foo_bars" ("foo_id", "bar_id") VALUES (1, 1)
INSERT INTO "app_foo_bars" ("foo_id", "bar_id") VALUES (1, 2)

让我们尝试分配 pk 的列表，正如 saverio 建议的那样:

Let's try to assign a list of pks, as saverio suggested:

foo = Foo()
foo.save()
foo.bars = [1,2]

由于我们没有获取两个Bar，所以我们保存了两个SELECT语句，结果一共5条:

As we don't fetch the two Bars, we save two SELECT statements, resulting in a total of 5:

INSERT INTO "app_foo" ("name") VALUES ()
SELECT "app_foo_bars"."id", "app_foo_bars"."foo_id", "app_foo_bars"."bar_id" FROM "app_foo_bars" WHERE "app_foo_bars"."foo_id" = 1
SELECT "app_foo_bars"."bar_id" FROM "app_foo_bars" WHERE ("app_foo_bars"."foo_id" = 1  AND "app_foo_bars"."bar_id" IN (1, 2))
INSERT INTO "app_foo_bars" ("foo_id", "bar_id") VALUES (1, 1)
INSERT INTO "app_foo_bars" ("foo_id", "bar_id") VALUES (1, 2)

获胜者是:

foo = Foo()
foo.save()
foo.bars.add(1,2)

将 pks 传递给 add() 给我们总共 4 个查询:

Passing pks to add() gives us a total of 4 queries:

INSERT INTO "app_foo" ("name") VALUES ()
SELECT "app_foo_bars"."bar_id" FROM "app_foo_bars" WHERE ("app_foo_bars"."foo_id" = 1  AND "app_foo_bars"."bar_id" IN (1, 2))
INSERT INTO "app_foo_bars" ("foo_id", "bar_id") VALUES (1, 1)
INSERT INTO "app_foo_bars" ("foo_id", "bar_id") VALUES (1, 2)

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