Entity Framework Core 零或一到零或一关系 [英] Entity Framework Core zero-or-one to zero-or-one relation
问题描述
给定这些类:
public class A
{
public int Id {get; set;}
public int? BId {get; set;}
public B B {get; set;}
}
public class B
{
public int Id {get; set;}
public int? AId {get; set;}
public A A {get; set;}
}
然后使用 Fluent API
Then with Fluent API
modelBuilder.Entity<A>()
.HasOne(a => a.B)
.WithOne(b => b.A)
.HasForeignKey<A>(a => a.BId);
创建对象并将它们添加到数据库时，相应表中的内容如下所示:
When creating objects and add them to database, things look like following in the corresponding tables:
- [A].BId 已设置
- [B].AId = null
当我使用 EF Core 检索数据时:
When I retrieve data using EF Core:
- A.B 已设置，A.Bid 已设置
- B.A 已设置，但 B.AId 为空.
我应该怎么做才能同时设置 B.AId?
What should I do to have B.AId set as well?
推荐答案
这些 0..1 : 0..1 关系通常定义在没有明显主体实体的实体之间.我喜欢汽车和司机的例子，这比 A 和 B 更容易想象.
These 0..1 : 0..1 relations are usually defined between entities of which none is an obvious principal entity. I like the example of cars and drivers, which is a bit more imaginable than A and B.
您所追求的模型如下所示:
The model you're after looks like this:
有两个相互的外键，它们都有一个唯一的索引，可以在数据库级别强制执行 1:1.
There are two mutual foreign keys, both of which have a unique index to enforce 1:1 at the database level.
HasOne - WithOne 组合不能在这里使用，因为它总是需要一个 HasForeignKey 指令来告诉哪个实体是主体.这也仅将一个字段配置为外键.在您的示例中， B.AId 只是一个常规字段.如果你不给它一个值，EF 也不会.
The HasOne - WithOne combi can't be used here, because that always requires a HasForeignKey instruction to tell which entity is principal. This also configures only one field as foreign key. In your example, B.AId is just a regular field. If you don't give it a value, EF won't either.
上述模型的映射比HasOne - WithOne要麻烦一些:
The mapping of the above model is a bit more cumbersome than HasOne - WithOne:
var carEtb = modelBuilder.Entity<Car>();
var driverEtb = modelBuilder.Entity<Driver>();
carEtb.HasOne(c => c.Driver).WithMany();
carEtb.HasIndex(c => c.DriverID).IsUnique();
driverEtb.HasOne(d => d.Car).WithMany();
driverEtb.HasIndex(c => c.CarID).IsUnique();
因此有两个 0..1:n 关联，它们通过外键上的索引变得唯一.
So there are two 0..1:n association that are made unique by indexes on the foreign keys.
创建以下数据库模型:
CREATE TABLE [Drivers] (
[ID] int NOT NULL IDENTITY,
[Name] nvarchar(max) NULL,
[CarID] int NULL,
CONSTRAINT [PK_Drivers] PRIMARY KEY ([ID])
);
CREATE TABLE [Cars] (
[ID] int NOT NULL IDENTITY,
[Brand] nvarchar(max) NULL,
[Type] nvarchar(max) NULL,
[DriverID] int NULL,
CONSTRAINT [PK_Cars] PRIMARY KEY ([ID]),
CONSTRAINT [FK_Cars_Drivers_DriverID] FOREIGN KEY ([DriverID])
REFERENCES [Drivers] ([ID]) ON DELETE NO ACTION
);
CREATE UNIQUE INDEX [IX_Cars_DriverID] ON [Cars] ([DriverID])
WHERE [DriverID] IS NOT NULL;
CREATE UNIQUE INDEX [IX_Drivers_CarID] ON [Drivers] ([CarID])
WHERE [CarID] IS NOT NULL;
ALTER TABLE [Drivers] ADD CONSTRAINT [FK_Drivers_Cars_CarID] FOREIGN KEY ([CarID])
REFERENCES [Cars] ([ID]) ON DELETE NO ACTION;
它创建了两个可空的外键，它们都由一个唯一的过滤索引索引.完美！
It creates two nullable foreign keys both indexed by a unique filtered index. Perfect!
可是……
EF 不认为这是双向的一对一关系.理所当然.两个 FK 就是这样，两个独立的 FK.但是，从数据完整性的角度来看，这种关系应该由两端建立:如果司机认领汽车(设置driver.CarID)，汽车也应该附加到司机(设置car.DriverID)，否则其他驱动程序可能会连接到它.
EF doesn't see this as a bidirectional one-on-one relationship. And rightly so. The two FKs are just that, two independent FKs. However, in view of data integrity the relationship should be established by both ends: if a driver claims a car (sets driver.CarID), the car should also be attached to the driver (set car.DriverID), otherwise another driver could be connected to it.
当现有的汽车和司机耦合时，可以使用一个小助手方法，例如在Car中:
When existing car and drivers are coupled a little helper method could be used, for example in Car:
public void SetDriver(Driver driver)
{
Driver = driver;
driver.Car = this;
}
然而，当 Car 和 Driver 在一个进程中和被创建和关联时，这是很笨拙的.EF 会抛出一个 InvalidOperationException:
However, when both a Car and Driver are created and associated in one process, this is clumsy. EF will throw an InvalidOperationException:
无法保存更改，因为在要保存的数据中检测到循环依赖关系:'Car [Added] <- Car { 'CarID' } Driver [Added] <- Driver { 'DriverID' } Car [Added]]'.
Unable to save changes because a circular dependency was detected in the data to be saved: 'Car [Added] <- Car { 'CarID' } Driver [Added] <- Driver { 'DriverID' } Car [Added]'.
意思是:一个FK可以一次性设置，另一个只能在保存数据后设置.这需要两个 SaveChanges 调用，在一段非常命令式的代码中包含在一个事务中:
Which means: one of the FKs can be be set at once, but the other one can only be set after saving the data. That requires two SaveChanges calls enclosed by a transaction in a pretty imperative piece of code:
using (var db = new MyContext())
{
using (var t = db.Database.BeginTransaction())
{
var jag = new Car { Brand = "Jaguar", Type = "E" };
var peter = new Driver { Name = "Peter Sellers", Car = jag };
db.Drivers.Add(peter);
db.SaveChanges();
jag.Driver = peter;
db.SaveChanges();
t.Commit();
}
}
替代方案:连接表
所以现在我之所以用这么长的篇幅解释所有这些:在我看来，0..1 : 0..1 关联应该由具有唯一外键的联结表建模:
Alternative: junction table
So now the reason why I go to these lengths explaining all this: in my opinion, 0..1 : 0..1 associations should be modeled by a junction table with unique foreign keys:
通过使用联结表 -
- 可以在原子操作中建立关联，而不是设置两个外键的容易出错的操作.
- 实体本身是独立的:它们没有外键，它们实际上不需要履行其职责.
这个模型可以通过这个类模型来实现:
This model can be implemented by this class model:
public class Car
{
public int ID { get; set; }
public string Brand { get; set; }
public string Type { get; set; }
public CarDriver CarDriver { get; set; }
}
public class Driver
{
public Driver()
{ }
public int ID { get; set; }
public string Name { get; set; }
public CarDriver CarDriver { get; set; }
}
public class CarDriver
{
public int CarID { get; set; }
public Car Car { get; set; }
public int DriverID { get; set; }
public virtual Driver Driver { get; set; }
}
和映射:
protected override void OnModelCreating(ModelBuilder modelBuilder)
{
var carDriverEtb = modelBuilder.Entity<CarDriver>();
carDriverEtb.HasKey(cd => new { cd.CarID, cd.DriverID });
carDriverEtb.HasIndex(cd => cd.CarID).IsUnique();
carDriverEtb.HasIndex(cd => cd.DriverID).IsUnique();
}
现在可以通过一个 SaveChanges 调用轻松创建驾驶员和汽车以及它们的关联:
Now creating drivers and cars and their associations can easily be done in one SaveChanges call:
using (var db = new MyContext(connectionString))
{
var ford = new Car { Brand = "Ford", Type = "Mustang" };
var jag = new Car { Brand = "Jaguar", Type = "E" };
var kelly = new Driver { Name = "Kelly Clarkson" };
var peter = new Driver { Name = "Peter Sellers" };
db.CarDrivers.Add(new CarDriver { Car = ford, Driver = kelly });
db.CarDrivers.Add(new CarDriver { Car = jag, Driver = peter });
db.SaveChanges();
}
唯一的缺点是从 Car 导航到 Driver vv 有点不方便.好吧，您自己看看哪种型号最适合您.
The only drawback is that navigting from Car to Driver vv is a bit les convenient. Well, see for yourself which model suit you best.
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