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.
The model you're after looks like this:
There are two mutual foreign keys, both of which have a unique index to enforce 1:1 at the database level.
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.
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();
So there are two 0..1:n association that are made unique by indexes on the foreign keys.
Which creates the following database model:
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;
It creates two nullable foreign keys both indexed by a unique filtered index. Perfect!
But...
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.
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;
}
However, when both a Car and Driver are created and associated in one process, this is clumsy. EF will throw an InvalidOperationException:
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]'.
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();
}
}
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:
By using a junction table -
- The association can be established in an atomic operation instead of an error-prone operation of setting two foreign keys.
- The entities themselves are independent: they don't have foreign keys they don't really need to fulfill their role.
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; }
}
And the mapping:
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();
}
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();
}
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.
