NHibernate IUserType获取映射的属性名称

Question

We are working on an NHibernate project where we needed to implement the NHibernate IUserType interface. So we did and so far it works fine (we haven't gotten around to testing the NullSafeSet method yet). However, we have 1 problem: We need to know what property we are mapping in order to be able to return the correct result.

Situation:

The value which we need to map is stored in the DB as an int , we need to create a Code object from this value. To be able to create the Code object, we require a string alongside with the previously mentioned int . This string is the name of the property we are mapping.

I already discovered that the names parameter contains the NHibernate SQL alias of the property (it was something in the form of: "IS69_6_") but I don't know whether or not this value stays the same (When i debugged it seemed to remain the same).

Some code to clarify: CodeTypeMapper.cs

public class CodeTypeMapper : IUserType
{
    public new bool Equals(object x, object y)
    {
        return x != null && x.Equals(y);
    }

    public int GetHashCode(object x)
    {
        return x.GetHashCode();
    }

    /********** Code relevant to the question: **********/
    public object NullSafeGet(IDataReader rs, string[] names, object owner)
    {
        var key = (int?)NHibernateUtil.Int32.NullSafeGet(rs, names[0]);
        if (key == null) return null;
        // So here we need to be able to get the correct code.
        // In order to get the correct code, we need the key (fetched above)
        // but we also need the name of the property that we are mapping
        // which has a dummy value in this example.
        Code code = MasterMetaProvider.MasterMeta.GetCode("PropertyName", (int)key);
        if (code == null) return null;
        // Now that we have the Code object, we can use it to return the proper result
        var result = new NetworkObjectEnum((int)key, code.CodeDescEn);
        return result;
    }
    /***************************************************/

    public void NullSafeSet(IDbCommand cmd, object value, int index)
    {
        if (value == null)
        {
            NHibernateUtil.Int32.NullSafeSet(cmd, null, index);
        }
        else
        {
            value = int.Parse(value.ToString());
            NHibernateUtil.Int32.NullSafeSet(cmd, value, index);
        }
    }

    public object DeepCopy(object value)
    {
        return value;
    }

    public object Replace(object original, object target, object owner)
    {
        return original;
    }

    public object Assemble(object cached, object owner)
    {
        return cached;
    }

    public object Disassemble(object value)
    {
        return value;
    }

    public SqlType[] SqlTypes
    {
        get
        {
            var types = new SqlType[1];
            types[0] = new SqlType(DbType.String);
            return types;
        }
    }
    public Type ReturnedType
    {
        get { return typeof(NetworkObjectEnum); }
    }
    public bool IsMutable
    {
        get { return false; }
    }
}

The NHibernate Mapping code:

// We need this Property's name
Map(x => x.PropertyName).Column("PropertyNameInDB").CustomType<CodeTypeMapper>();

---

Question:

So the question I want to ask: Is it possible to access the above property's name in the CodeTypeMapper class ?

Thanks in advance.

Regards, Yanik Ceulemans

---

Update 23 Sept 2013:

As an attempt to clarify my question, I will give an example: Say there is a code table that specifies cities, types of transport, etc...:

+----+-----------+------------+-----------+
| id | code_type | code_value | code_desc |
+----+-----------+------------+-----------+
|  1 | city      |          5 | Antwerp   |
|  2 | city      |          8 | Brussels  |
|  3 | city      |          1 | Ghent     |
|  4 | transport |          1 | boat      |
|  5 | transport |          8 | plane     |
|  6 | transport |          9 | car       |
+----+-----------+------------+-----------+

Next, we have a travel table that uses this code table in the following fashion:

+----+------+-------------------+-----------+
| id | city |   other_column    | transport |
+----+------+-------------------+-----------+
|  1 |    5 | some random value |         1 |
|  2 |    8 | randomness        |         8 |
|  3 |    1 | a value           |         9 |
+----+------+-------------------+-----------+

As you can see: if we want to know the description for a code, we know what value we need to take from the code table by looking at the column name, and the value in that column from the travel table.

The c# class Travel will probably look something like this:

public class Travel
{
    public int Id { get; set; }
    public int City { get; set; }
    public string OtherColumn { get; set; }
    public int Transport { get; set; }
}

So my question is: Is it possible when mapping the Travel class, to get the property name "city" inside the IUserType implementation at runtime in order to be able to grab the correct value from the code table ? (note: I cannot change the DB design.)

Answer 1

This design is known as a "god table" and you have my sympathies. Our main product had one of these when I started and it took me a year to normalize it.

Before the refactoring, I mapped this with NHibernate by using views to virtually normalize it. That is, create a view for city, transport, etc. and map those as if they were tables. I see that you can't change the schema but if you can add views this is a good solution.

Alternative 1: Create an IUserType implementation for each type in your code table.

Alternative 2: Prior to using views, I used NHibernate's ability to add a where clause to the mapping. I remember that there were drawbacks to this approach, one of which I think was that the where clause was not always applied as expected in queries. Using Fluent NHibernate the mapping would look like this:

public abstract Class CodeTableBase
{
    public virtual int Id { get; protected set; }
    public virtual string CodeType { get; protected set; }
    public virtual int Value { get; protected set; }
    public virtual string Description { get; protected set; }
}

public class City : CodeTableBase {}

public abstract class CodeTableMap<T> : ClassMap<T> where T : CodeTableBase
{
    public CodeTableMap() 
    {
        Table("CodeTable");
        Id(x => Id, "id").your id generation strategy
        CodeType(x => x.CodeType, "code_type");
        Value(x => x.Value, "code_value");
        Description(x => x.Description, "code_description");
    }

    public CodeTableMap(string codeType) : this()
    {
        Where("code_type = '" + codeType + '"); // may need to use property name
    }
}

public class CityMap : CodeTableMap<City>
{
    public CityMap : base("city") {}
}

Answer 2

I'm not sure what you mean, but maybe you need something like this:

Here

---

Implementing IParameterizedType you can send in mapping the name of the property:

Here

Fluent NHibernate does not provide capability to this but you can use xml files.

Mapping

---

Other not much elegant solution:

var propertyName = GetPropertyName(owner.GetType(), owner, typeof(YourAttribute));

public static string GetPropertyName(Type entityType, object entity, Type attributeType)
{
    if (entity == null) return null;

    var propertyInfo = GetPropertyInfo(entityType, attributeType);

    return propertyInfo == null ? null : propertyInfo.Name;
}

public static PropertyInfo GetPropertyInfo(Type entityType, Type attributeType)
{
    return entityType.GetProperties().FirstOrDefault(prop => prop.GetCustomAttributes(false)
        .Count(x => x.GetType() == attributeType) > 0);
}