How to map a bitwise sum to a list in NHibernate

Question

I have a field in my database that stores an integer value and contains the sum of the bitwise values selected in a list. For instance:

VALUE DESCRIPTION
----- -----------
  1    Option 1
  2    Option 2
  4    Option 3
  8    Option 4

Let's say Options 2 & 4 are selected, so the value stored in the field would be 10.

I'm having a hard time figuring out (if it's even possible) how to represent this in the hbm.xml file.

Here is a generic example of what I'm trying to do:

Product Table Columns

Id, int
Name, varchar(25)
Services, int

Service Table Columns

Id, int
Name, varchar(25)

Product.cs

public class Product
{
    public virtual int Id { get; set; }
    public virtual string Name { get; set; }
    public virtual IList<Service> Services { get; set; }
}

Service.cs

public class Service
{
    public virtual int Id { get; set; } // bit values: 1, 2, 4, 8, ...
    public virtual string Name { get; set; }
}

Product.hbm.xml

<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2" assembly="AppNS" namespace="AppNS">
    <class name="Product" table="Product">
        <id name="Id" column="Id" type="int">
            <generator class="native"/>
        </id>
        <property name="Name" column="Name" type="string"/>
        <????? name="Services" column="Services" type="AppNS.Service"/>
    </class>
</hibernate-mapping>

Service.hbm.xml

<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2" assembly="AppNS" namespace="AppNS">
    <class name="Service" table="Service">
        <id name="Id" column="Id" type="int">
            <generator class="native"/>
        </id>
        <property name="Name" column="Name" type="string"/>
    </class>
</hibernate-mapping>

I need help with the <????> part in the Product.hbm.xml file.

--EDIT--

Ultimately, I want to be able to call the Load() method to get my Product model back.

Configuration cfg = new Configuration();
...
ISessionFactory sf = cfg.BuildSessionFactory();
using (ISession s = sf.OpenSession())
{
    Product product = s.Load<Product>(100);
    foreach(Service service in product.Services)
    {
        Console.WriteLine(service.Name);
    }
}

Output would be:

Option 2
Option 4

Answer 1

Such field should be mapped as a simple property. The entity should type this property as a flag enum. I do not think you can map that directly as a list.

You may instead compute your list from the mapped enum.

[Flags]
public enum EService
{
    Option1 = 1,
    Option2 = 2,
    Option3 = 4,
    Option4 = 8
}

public class Product
{
    public virtual int Id { get; set; }
    public virtual string Name { get; set; }
    public virtual EService Services { get; set; }

    // Coded here for simplicity, though you'd probably do not want such
    // dependencies here.
    public virtual List<Service> GetServicesEntities(ISession session)
    {
        var services = new List<Service>();
        foreach (EService s in Enum.GetValues(typeof(EService)))
        {
            if ((Services & s) != 0 &&
                // In case you predefine combinations of options in the enum, you need 
                // this to avoid having them in the list too.
                IsPowerOfTwo((int)s))
            {
                services.Add(
                    // Not a n+1 perf trouble if you have lazy loading batching enabled.
                    session.Load<Service>(s));
            }
        }
        return services;
    }

    // Taken from http://stackoverflow.com/a/600306/1178314
    private bool IsPowerOfTwo(int x)
    {
        return (x != 0) && ((x & (x - 1)) == 0);
    }
}

<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2" assembly="AppNS" namespace="AppNS">
    <class name="Product" table="Product">
        <id name="Id" column="Id">
            <generator class="native"/>
        </id>
        <property name="Name" />
        <property name="Services" />
    </class>
</hibernate-mapping>

(Simplified the mapping by the way, NHibernate assumes columns names are same as property names if not specified, and infers property types from the entity.)

Answer 2

Try xml linq

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Xml;
using System.Xml.Linq;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            string header = 
                "<?xml version=\"1.0\" encoding=\"utf-8\" ?>" + 
                "<hibernate-mapping xmlns=\"urn:nhibernate-mapping-2.2\" assembly=\"AppNS\" namespace=\"AppNS\">" +
                "</hibernate-mapping>";

            Product.products = new List<Product>() {
                new Product() { Id = 100, Name = "Product", Services = new List<Service>() {
                    new Service() {Id = 1, Name = "Option 1"},
                    new Service() {Id = 2, Name = "Option 2"},
                    new Service() {Id = 4, Name = "Option 3"},
                    new Service() {Id = 8, Name = "Option 4"}
                }}
            };

            XDocument serviceXml = XDocument.Parse(header);

            XElement mapping = (XElement)serviceXml.FirstNode;
            foreach(Product product in Product.products)
            {
                XElement newProduct = new XElement("class");
                mapping.Add(newProduct);

                newProduct.Add(new object[] {
                    new XAttribute("name",product.Name),
                    new XAttribute("table","Product"),
                    new XElement("id", new object[] {
                        new XAttribute("name", product.Id),
                        new XAttribute("column", product.Id),
                        new XAttribute("type","int"),
                        new XElement("generator", new XAttribute("class","native"))
                    }),
                    new XElement("property", new object[] {
                        new XAttribute("name","Name"),
                        new XAttribute("column", "Name"),
                        new XAttribute("type","string")
                    })
                });
                XElement services = new XElement("class", new object[] {
                    new XAttribute("name", "Service"),
                    new XAttribute("table", "Service")
                });
                newProduct.Add(services);

                foreach (Service service in product.Services)
                {
                    services.Add(new XElement("id", new object[] {
                        new XAttribute("name", service.Name),
                        new XAttribute("column", service.Id),
                        new XAttribute("type", "int"),
                        new XElement("generator", new XAttribute("class","native"))
                    }));
                }
            }
        }
    }
    public class Product
    {
        public static List<Product> products = new List<Product>();
        public virtual int Id { get; set; }
        public virtual string Name { get; set; }
        public virtual IList<Service> Services { get; set; }
    }
    public class Service
    {
        public virtual int Id { get; set; } // bit values: 1, 2, 4, 8, ...
        public virtual string Name { get; set; }
    }
}