Structural Typing Edit on GitHub

Let's say that you have some kind of complicated document type that represents the single source of truth for that information within your system, but you frequently need to fetch only a subset of that document. In that particular case you can opt into Marten's (limited) support for "structural typing."

See these two document types:

namespace Area1
{
    public class Product
    {
        public int Id { get; set; }

        public string Name { get; set; }

        public decimal Price { get; set; }
    }
}

namespace Area2
{
    [StructuralTyped]
    public class Product
    {
        public int Id { get; set; }

        public string Name { get; set; }
    }
}

The Area2.Product has a subset of the properties that Area1.Product has. By marking Area2.Product with the [StructuralTyped] attribute, we are directing Marten to pull Area2.Product data from the underlying storage of the bigger Area1.Product document.

You can see this in action inside of a unit test from Marten:

[Fact]
public void can_select_from_the_same_table()
{
    using (var session = theStore.OpenSession())
    {
        var product1 = new Area1.Product { Name = "Paper", Price = 10 };
        session.Store(product1);
        session.SaveChanges();

        var product2 = session.Load<Area2.Product>(product1.Id);

        product2.Name.ShouldBe(product1.Name);
    }
}

The caveats here are that the document types must have the same name, cannot be inner classes, must be in separate namespaces, and the underlying JSON serializer must be able to resolve the structural typed subset documents from the raw JSON of the master document.