关于sql:EF-core关联对象删除

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DeleteBehavior

@想一想 @:在关联的 entity 之间,当删除父 entity(比方 Classroom)的时候,子 entity(比方 Student)应该如何解决?

温习:__SQL 中的 Cascade Delete

自定义:OnDelete() 办法

在 OnModelCreating() 中可调用 OnDelete() 办法,传递 DeleteBehavior 枚举值,指定 EF core 如何操作。

比方:

modelBuilder.Entity<Student>() .HasOne<Bed>(s => s.SleepIn) .WithOne(b => b.Owner) .HasForeignKey<Student>(s => s.SleepInId) .OnDelete(DeleteBehavior.Cascade) ;

EF 的 DeleteBehavior 枚举能够被归为 3 类。对应着当删除父 entity 的时候,子 entity 或数据行的 3 种解决计划:

  1. Cascade/ClientCascade:子 entity 被同时删除
  2. SetNull/ClientSetNull:其外键被设置为 null(如果外键不能被设置为 null,会报异样)
  3. NoAction:不产生扭转

Client 和非 Client

比方 DeleteBehavior.SetNull 和 DeleteBehavior.ClientSetNull:

  • client 指 EF,即:EF 会生成解决子 entity 的 Update/Delete 语句,交数据库执行,不依赖于数据库的设定
  • 非 client,需依附数据库上的设定,EF 不会生成解决子 entity 的 Update/Delete 语句

所以,如果设置是:

  • Client:子 entity 要曾经退出 DbContext(比方 1:n 的时候,n 要双向援用且被加载到 DbContext)
  • 非 Client:数据库要有相应的 Cascade 设置。如果通过 Migration 或 EnsureCreated 创立的数据库,数据库上的 Cascade 会主动配置……

演示:

  1. 应用 EF 的 migration 或者 API 建库建表,不同的 DeleteBehavior 生成不同的 SQL 建表语句,比方:

    1. __DeleteBehavior.SetNull,生成 ON DELETE SET NULL

      ALTER TABLE [dbo].[Students] ADD CONSTRAINT [FK_Students_Bed_SleepInId] FOREIGN KEY ([SleepInId]) REFERENCES [dbo].[Bed] ([Id]) ON DELETE __SET NULL__;

      __

    2. DeleteBehavior.ClientCascade,没有 ON DELETE 后缀(_@想一想 @_:为什么?)
  2. 删除一个父 entity,在 Logger 中查看是否有删除 / 更新子 entity 的 SQL 语句

留神: 如果说 EF 的 Cascade 配置和数据库设置相冲突,会报异样。

默认行为

EF 依据子 entity 是否必须要有一个父 entity(即数据库上外键是否能够为 Null),默认设置 Behavior:

  • 如果外键能够为空:DeleteBehavior.ClientSetNull
  • 如果外键不能够为空:DeleteBehavior.Cascade

标记删除

当 entity 之间的关系简单之后,级联删除会导致简单的链式反应,比方:

删除 A 就会删除 B,删除 B 就会删除 C 和 D,删除 D 又会删除 E 和 F……但并不是每个 entity 都能删得掉

所以咱们通常都不进行物理删除,而是用 flag 标记删除(在 entity 中增加一个是否已删除的 Flag 列),最初再由 DBA 批量删除,其劣势为:

  • 缩小用户等待时间
  • 能够回溯撤销
  • 便于管制意外状况产生

删除关系

有时候咱们并不需要删除 entity,只是想“切断”entity 之间的关系。比方:1 号教室原来是 lw、zl、lzb 三个同学,当初将 lzb 移出该教室(而不是删除)。

留神首先要把关联 entity 加载到 DbContext 中来。

SetNull 和 Remove

如果援用是单向的话,无论关联 entity 是汇合还是单个元素。

小 bug

应用 LazyLoad,比方在“多对多”关系中,你可能会认为这样的代码是“从新赋值”

但查看运行后果,居然是“增加”了。这是怎么一回事呢?https://github.com/hdfgfgh546…
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