WCF Data Services (OData) - Expand navigation properties using foreign key
public class Entitlement : IEntity
{
    #region Implementation of IEntity
    public string Id { get; set; }
    #endregion

    public string ItemId { get; set; }

    [ForeignProperty]
    public Item Item { get; set; }
}

public class Item : IEntity
{
    #region Implementation of IEntity
    public string Id { get; set; }
    #endregion

    public string ItemName { get; set; }
}