Support for strongly-typed IDs

[adrianfrankowski95]

Hello!

First of all I would like to thank all contributors, I really find this project very unique and interesting!

I encountered an issue regarding strongly-typed IDs. In my project I use DDD and create custom classes for IDs, like in this example:

public class PostId : ValueObject<PostId>
{
    public Guid Value { get; }

    public PostId(Guid value)
    {
        Value = value;
    }

    protected override IEnumerable<object> GetEqualityCheckAttributes()
    {
        yield return Value;
    }
}

And consume it in a following way:

public abstract class Entity<T>
{
    public T Id { get; protected set; }
}

public class PostBase : Entity<PostId>
{
    ...
}

Then I'm trying to tell Marten which value is an ID:

builder.Services.AddMarten(opt =>
{
    opt.Schema.For<PostBase>()
        .Identity(x => x.Id.Value);
});

But unfortunately such setup results in an ArgumentOutOfRangeException:

System.Private.CoreLib.dll: 'Id members must be an int, long, Guid, or string'

In EF Core I could easily provide convertion to/from GUID:

public virtual void Configure(EntityTypeBuilder<PostBase> builder)
{
        builder
            .Property(x => x.Id)
            .HasConversion(x => x.Value, x => new PostId(x))
            .HasMaxLength(68);
}

Is there any workaround for this issue or shall I just use pure GUID?

Thank you!

[jeremydmiller]

Heh, I just closed the last issue someone opened for strongly typed ids. I'm against doing this in Marten tbh, because I just don't want the extra complexity. You could possibly use this as is with Marten by making your strong typed id types be implicitly convertible to the Marten supported identifier types like an int, long, string, or Guid.

[adrianfrankowski95]

Thank you for your answer. I've just tried to implement implicit/explicit conversions to/from Guid and unfortunately I still cannot make it work:

public static implicit operator Guid(PostId id) => id.Value;
public static implicit operator PostId(Guid id) => new(id);

And in registering Marten I tried the following ways:

.Identity(x => x.Id);
.Identity(x => x.Id.Value);
.Identity(x => (Guid)x.Id);

And I still get the same exception. I also tried to convert it to string with no luck.
Could you please share working example of what kind of conversion is expected?

BTW: I'm using Marten 5.0.0-alpha.4

[jeremydmiller]

In that case, I don't think there is a workable solution with the existing Marten and you'll have to revert to using primitive types for the Id.

And in your Identity(expression) usage, Marten is only picking up the property or field and doesn't know how to do any kind of magic casting at runtime.