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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