RIA Services: Sending complex types to the client

Entity Framework 4 allows us to create complex types. Typically, such types are used to get the result of a stored procedure. However, when we try to send such a complex type using a WCF RIA Domain Service, the following error is encountered.

The entity ' StoredProcedure_Result' in DomainService 'MyDomainService' does not have a key defined. Entities exposed by DomainService operations must have at least one public property marked with the KeyAttribute.

What to do? Adding attributes to EF designer generated classes is a bad idea; Also, we cannot use partial classes to add attributes to an already defined property. However, luckily, WCF RIA Services uses metadata classes in conjunction with the entity model classes and hence we can always create a meta data class for our complex type and decorate one of the fields with a [Key] attribute. E.g., If our stored procedure is returning StoredProcedure_Result, then we may just need to add the following in our DomainService.metadata.cs file:

[MetadataTypeAttribute(typeof(StoredProcedure_Result.StoredProcedure_Result_Metadata))]
public partial class StoredProcedure_Result
{
    internal sealed class StoredProcedure_Result_Metadata
    {
        // Metadata classes are not meant to be instantiated, so mark constructor as private
        private StoredProcedure_Result_Metadata()
        { }
 
        [Key]
        public string SomeUniqueColumn { get; set; }
    }
}

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