Cannot create composite key from class

[sam-wheat]

Many of the classes in my project have a PK composed of an int and a string property. I would like to move those two properties to their own class and use that class as the key on my entities:

Identifier:

    public class NativeIdentifier
    {
        public int DataProviderID { get; set; }
        public string NativeID { get; set; }

    }

Entities:

    public class Category
    {
        public NativeIdentifier NativeID { get; set; }
        public string Name { get; set; }

        public virtual DataProvider DataProvider { get; set; }
        public virtual ICollection<SeriesCategory> SeriesCategories { get; set; }
    }

    public class Series
    {
        public NativeIdentifier NativeID { get; set; }
        public string Name { get; set; }

        public DataProvider DataProvider { get; set; }
        public virtual ICollection<SeriesCategory> SeriesCategories { get; set; }
    }

    public class SeriesCategory
    {
        public NativeIdentifier SeriesNativeID { get; set; }
        public NativeIdentifier CategoryNativeID { get; set; }

        public virtual Series Series { get; set; }
        public virtual Category Category { get; set; }
    }

The entity I am having a problem with is SeriesCategory which simply joins Series and Category.

I try to define the PK as shown below. I need to name the properties because there are duplicate names:

    mb.Entity<SeriesCategory>().HasKey(x => new { CatDPID = x.CategoryNativeID.DataProviderID, CatNativeID = x.CategoryNativeID.NativeID, SeriesDPID = x.SeriesNativeID.DataProviderID, SeriesNativeID = x.SeriesNativeID.NativeID });

I get this error:

The properties expression 'x => new <>f__AnonymousType0`4(DataProviderID = x.CategoryNativeID.DataProviderID, NativeID = x.CategoryNativeID.NativeID, SeriesDPID = x.SeriesNativeID.DataProviderID, SeriesNativeID = x.SeriesNativeID.NativeID)' is not valid. The expression should represent a simple property access: 't => t.MyProperty'. When specifying multiple properties use an anonymous type: 't => new { t.MyProperty1, t.MyProperty2 }'.
Parameter name: propertyAccessExpression

Is there a way to accomplish this?

[ajcvickers]

Note for triage: this should work once #13947 is implemented, but we may want to keep this open to track the specific case of using this for a key.