实体框架在子类上是唯一的

Question

I am confused about how this kind of situation should be handled in Entity Framework.

I have a character base class:

public class Character
{
    [Key]
    public Guid Id { get; set; }
    public Realm Realm { get; set; }
    public Guid RealmId { get; set; }
    public string Name { get; set; }
}

Character has name and is bound to realm.

Character controlled by a player is definitely a character. It is bound to a player/user:

public class PlayerCharacter : Character
{        
    public User User { get; set; }
    public Guid UserId { get; set; }
}

NPC is also a character: it does not need to have any properties for a MCVE

public class Npc : Character { }

Now, obviously, you would not want to have to characters on the same realm with the same name, but you can have as many Orcs as you need on a single realm, so we would need to define unique constraint on PlayerCharacter , for example via .HasAlternateKey :

public DbSet<PlayerCharacter> Characters { get; set; }
public DbSet<Npc> Npcs { get; set; }

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    base.OnModelCreating(modelBuilder);

    modelBuilder.Entity<PlayerCharacter>()
        .HasAlternateKey("Name", "RealmId");
}

Now we add migration:

$ dotnet ef migrations add 5
System.InvalidOperationException: A key cannot be configured on 
'PlayerCharacter' because it is a derived type. The key must be 
configured on the root type 'Character'.

Ok, how about a unique index?

modelBuilder.Entity<PlayerCharacter>()
    .HasIndex("Name", "RealmId")
    .IsUnique();

$ dotnet ef migrations add 5
Done. To undo this action, use 'ef migrations remove'

It worked! But wait:

migrationBuilder.CreateTable(
    name: "Character",
    columns: table => new
    {
        Id = table.Column<Guid>(nullable: false),
        Discriminator = table.Column<string>(nullable: false),
        Name = table.Column<string>(nullable: true),
        RealmId = table.Column<Guid>(nullable: false),
        // other props...
    },
    constraints: table =>
    {
        table.PrimaryKey("PK_Character", x => x.Id);
        // foreign keys...
    });

migrationBuilder.CreateIndex(
    name: "IX_Character_Name_RealmId",
    table: "Character",
    columns: new[] { "Name", "RealmId" },
    unique: true);

Now both player characters and npcs are unique by name! How can I set unique only in derived class?

Answer 1

The only way you can do it currently (EF Core 2.0.1) is to use filtered unique index based on Discriminator column value.

Unfortunately the HasFilter fluent API is not db agnostic. Here is the configuration for SqlServer:

modelBuilder.Entity<PlayerCharacter>()
    .HasIndex("Name", "RealmId")
    .IsUnique()
    .HasFilter("Discriminator = 'PlayerCharacter'"); // <--