Set MaxLength on TPH discriminator property by convention

[satyajit-behera]

The Discriminator column in TPH is created nvarchar(max) by default.

Can we set it to a fixed length (say nvarchar(200)) and index it globally with one setting.

Thanks

[ajcvickers]

@satyajit-behera I just sent a PR to update the docs for it: dotnet/EntityFramework.Docs#601. Here's the update:

Configuring the discriminator property

In the examples above, the discriminator is created as a shadow property on the base entity of the hierarchy. Since it is a property in the model, it can be configured just like other properties. For example, to set the max length when the default, by-convention discriminator is being used:

modelBuilder.Entity<Blog>()
    .Property("Discriminator")
    .HasMaxLength(200);

The discriminator can also be mapped to an actual CLR property in your entity. For example:

class MyContext : DbContext
{
    public DbSet<Blog> Blogs { get; set; }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Blog>()
            .HasDiscriminator<string>("BlogType");
    }
}

public class Blog
{
    public int BlogId { get; set; }
    public string Url { get; set; }
    public string BlogType { get; set; }
}

public class RssBlog : Blog
{
    public string RssUrl { get; set; }
}

Combining these two things together it is possible to both map the discriminator to a real property and configure it:

modelBuilder.Entity<Blog>(b =>
{
    b.HasDiscriminator<string>("BlogType");

    b.Property(e => e.BlogType)
        .HasMaxLength(200)
        .HasColumnName("blog_type");
});

[satyajit-behera]

Thanks. This fixes for each entity individually. Is there any way where I can set it for all the discriminators across Context in one setting.

[ajcvickers]

@satyajit-behera Run through all the entity types at the end of OnModelCreating and add configuration. For example, something like this:

foreach (var entityType in modelBuilder.Model.GetEntityTypes())
{
    entityType.FindProperty("Discriminator")?.SetMaxLength(200);
}

[satyajit-behera]

@ajcvickers Thanks a lot

[Schuttrim]

I couldn't set the max Length using the b.Property(e => e.BlogType).HasMaxLength(200) syntax, ince I'm using the ToTable(string) Method which isn't available after using the Entity<TEntity>([NotNull] Action<EntityTypeBuilder<TEntity>> buildAction) Method, since it returns a ModelBuilder instead of EntityTypeBuilder.

Thats why I added the configuration right after with the following code:

modelBuilder.Model.GetEntityTypes(typeof(Blog)).Single().FindProperty("BlogType").SetMaxLength(200);

[ajcvickers]

Note from triage: consider using an approach like shown here: dotnet/EntityFramework.Docs#4000, possibly using power-of-two buckets to avoid a migration on every change to discriminator value length.