如何创建一对一关系，双方关系都是可选的

Question

Is it possible to create a relationship between two entities where each entity has a foreign key to the other and the foreign key fields also optional? I tried creating tables based on the following code:

public class Museum
{
    public int MuseumId { get; set; }
    public string Name { get; set; }

    [ForeignKey("Curator")]
    public int? CuratorId { get; set; }
    public virtual Curator Curator { get; set; }
}

public class Curator
{
    public int CuratorId { get; set; }
    public string Name { get; set; }

    [ForeignKey("Museum")]
    public int? MuseumId { get; set; }
    public virtual Museum Museum { get; set; }
}

But I get the following error:

Curator_Museum_Target: : Multiplicity is not valid in Role 'Curator_Museum_Target' in relationship 'Curator_Museum'. Because the Dependent Role properties are not the key properties, the upper bound of the multiplicity of the Dependent Role must be '*'.

Is there any way to get around this?

Answer 1

You can't do that in a database. A FK must refer to a PK on the other side of the relation. And a PK cannot be optional. For example, the relation that you want o implement between tables A and B would have these conditions:

• if A.Id is an FK related to B.Id , B.Id must be a PK, and thus, not optional
• and viceversa, if B.Id is an FK related to A.Id , A.Id must be a PK, and thus, not optional

So, this is not a problem of EF, but a problem of your model.

To simulate this you can use a bridge table, like this "pseducode":

[Table: ABBridge]
AId not null
BId not null

So, if there is a dependency form A to B or viceversa, you simply have to add an entry to this table. It there is no such relation, there is no field in this table. Both AId an BId should be the PK for the bridge table.

This would be a many-to-many relation. If you're using a modern version of EF (6.1 or later) you can convert it in a one-to-one by making both AId and BId unique by using the Index attribute with IsUnique in both of them.

[Index("AIdUniqueIndex", IsUnique = true)]
public int AId { get; set; }
[Index("BIdUniqueIndex", IsUnique = true)]
public int AId { get; set; }

If you are using an older version, you should ensure this using business logic.

Answer 2

You can configure this by using the fluent API of the ModelBuilder.

For an optional 1:1 relationship the code is:

ModelBuilder.Entity<Museum>().HasOptional(x => x.Curator).WithOptionalPrincipal(x => x.Museum).WillCascadeOnDelete(true);

As JotaBe noted EF will not create a foreign key column in both tables when mapping this config to the database. Instead the FK column will only be created for the side defined in the HasOptional() clause. The primary/unique key base table will be the side configured as WithOptionalPrincipal().

For the example code, the FK constraint on the DB would be created in the Curator table as Curator.Museum_Id -> Museum.Id, with Museum as the Primary/Unique key base table.

Also note that the code given above configures cascade delte as "delete the curator if the museum gets deleted".