Answers:
您不能从未保存的对象创建m2m关系。如果有pk
,请尝试以下操作:
sample_object = Sample()
sample_object.save()
sample_object.users.add(1,2)
更新:阅读了saverio的答案后,我决定对这个问题进行更深入的研究。这是我的发现。
这是我最初的建议。它可以工作,但不是最佳选择。(注意:我使用Bar
的是s和a Foo
而不是User
s和a Sample
,但是您知道了。)
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:
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该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:
bar1 = Bar.objects.get(pk=1)
bar2 = Bar.objects.get(pk=2)
foo = Foo()
foo.save()
foo.bars = [bar1, bar2]
不幸的是,这又增加了一个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)
让我们尝试分配一个pk
s 列表,如saverio建议的那样:
foo = Foo()
foo.save()
foo.bars = [1,2]
由于不获取两个Bar
s,因此保存了两个SELECT
语句,总共有5个:
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)
最终获胜者是:
foo = Foo()
foo.save()
foo.bars.add(1,2)
路过pk
s到add()
让我们一共有4个查询:
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)
对于将来的访问者,您可以使用django 1.4中新的bulk_create在2个查询中创建一个对象及其所有m2m对象。请注意,仅当您不需要对带有save()方法或信号的数据进行任何预处理或后处理时,此方法才可用。您插入的正是数据库中的内容
您无需在字段上指定“直通”模型即可执行此操作。为了完整起见,下面的示例创建了一个空白的Users模型来模仿原始海报的要求。
from django.db import models
class Users(models.Model):
pass
class Sample(models.Model):
users = models.ManyToManyField(Users)
现在,在Shell或其他代码中,创建2个用户,创建一个示例对象,然后将用户批量添加到该示例对象中。
Users().save()
Users().save()
# Access the through model directly
ThroughModel = Sample.users.through
users = Users.objects.filter(pk__in=[1,2])
sample_object = Sample()
sample_object.save()
ThroughModel.objects.bulk_create([
ThroughModel(users_id=users[0].pk, sample_id=sample_object.pk),
ThroughModel(users_id=users[1].pk, sample_id=sample_object.pk)
])
RelatedObjectManagers与Model中的字段是不同的“属性”。实现您想要的最简单的方法是
sample_object = Sample.objects.create()
sample_object.users = [1, 2]
这与分配用户列表相同,而没有其他查询和模型构建。
如果查询的数量让您感到困扰(而不是简单),那么最佳解决方案将需要三个查询:
sample_object = Sample.objects.create()
sample_id = sample_object.id
sample_object.users.through.objects.create(user_id=1, sample_id=sample_id)
sample_object.users.through.objects.create(user_id=2, sample_id=sample_id)
这将起作用,因为我们已经知道“用户”列表为空,因此我们可以轻松创建。