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 ）
  

我想这样做（在这个模型中保存用户，user1和user2） - >

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

$ b $我知道这是错的：D，但我相信你会得到我想做的事情，所以你怎么做？

解决方案

无法从未保存的对象创建m2m关系。如果你有pks，请尝试：

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

更新：之后阅读 saverio的答案，我决定深入调查这个问题。这是我的发现。

这是我原来的建议。它有效，但不是最佳的。 （注意：我使用栏和一个 Foo 而不是用户 s和一个样本，但你会得到想法）。

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

它总共产生7个查询：

  SELECTapp_bar。id，app_bar FROMapp_barWHEREapp_bar。id= 1 
 SELECTapp_bar。id，app_bar。nameFROMapp_barWHEREapp_bar。id= 2 
 INSERT INTOapp_foo（name）VALUES（）
 SELECTapp_foo_bars。bar_idFROMapp_foo_barsWHERE（app_foo_bars。foo_id= 1 ANDapp_foo_bars。bar_id IN（1））
 INSERT INTOapp_foo_bars（foo_id，bar_id）VALUES（1，1）
 SELECTapp_foo_barsbar_idFROMapp_foo_barsWHERE（app_foo_bars foo_id= 1 ANDapp_foo_bars。bar_idIN（2））
 INSERT INTOapp_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：

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

如我们所见，传递多个对象可以节省一个code> SELECT ：

  SELECTapp_bar。id，app_bar nameFROMapp_barWHEREapp_bar。id= 1 
 SELECTapp_bar。id，app_bar。nameFROMapp_barWHEREapp_barid= 2 
 INSERT INTOapp_foo（name）VALUES（）
 SELECTapp_foo_barsbar_idFROMapp_foo_barsWHERE（app_foo_bars。foo_id= 1 ANDapp_foo_bars。 bar_idIN（1，2））
 INSERT INTOapp_foo_bars（foo_id，bar_id）VALUES（1，1）
 INSERT INTOapp_foo_bars（foo_id，bar_id ）VALUES（1，2）
  

我不知道你也可以分配一个对象：

  bar1 = Bar.objects.get（pk = 1）
 bar2 = Bar.objects。 get（pk = 2）
 foo = Foo（）
 foo.save（）
 foo.bars = [bar1，bar2] 
  pre> 
 
 
不幸的是，创建一个额外的 SELECT ：
 
 
 < 从app_barWHEREapp_bar。id= 1 
 SELECTapp_bar。 id，app_bar。nameFROMapp_barWHEREapp_bar。id= 2 
 INSERT INTOapp_foo（name）VALUES（）
 SELECTapp_foo_bars 。id，app_foo_bars。foo_id，app_foo_bars。bar_idFROMapp_foo_barsWHEREapp_foo_barsfoo_id= 1 
 SELECTapp_foo_barsbar_idFROMapp_foo_bars WHERE（app_foo_bars。foo_id= 1 ANDapp_foo_bars。bar_idIN（1,2））
 INSERT INTOapp_foo_bars（foo_id，bar_id）VALUES（1，1） 
 INSERT INTOapp_foo_bars（foo_id，bar_id）VALUES（1，2）
  

  foo = Foo（）
我们尝试分配一个'pk'的列表， foo.save（）
 foo.bars = [1,2] 
  

由于我们没有获取两个 Bar s，所以我们保存两个 SELECT 语句，导致总共5：

  INSERT INTOapp_foo（name）VALUES（） 
 SELECTapp_foo_barsid，app_foo_barsfoo_id，app_foo_barsbar_idFROMapp_foo_barsWHEREapp_foo_barsfoo_id= 1 
 SELECTapp_foo_bars 。bar_idFROMapp_foo_barsWHERE（app_foo_bars。foo_id= 1 ANDapp_foo_bars。bar_idIN（1,2））
 INSERT INTOapp_foo_bars（foo_id，bar_id ）VALUES（1，1）
 INSERT INTOapp_foo_bars（foo_id，bar_id）VALUES（1，2）
  

获胜者是：

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

传递 pk s to add（）给我们总共4个查询：

  INSERT INTOapp_foo（name）VALUES（）
 SELECTapp_foo_barsbar_idFROMapp_foo_barsWHERE（app_foo_bars。foo_id= 1 ANDa pp_foo_barsbar_idIN（1，2））
 INSERT INTOapp_foo_bars（foo_id，bar_id）VALUES（1，1）
 INSERT INTOapp_foo_bars（foo_id ，bar_id）VALUES（1，2）
  

My model ->

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

I want to do this (save users, user1 and user2 in this model ) ->

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

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

解决方案

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)

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

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)

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)

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)

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)

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]

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)

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

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

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)

And the winner is:

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

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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