Map two classes to the same table

Question

There is a NODES table with dozen of 'small' columns and a LOB column in a legacy DB. A NodeEntity class is mapped to the NODES table.

For performance purposes I do not want to load LOB column every time I access the DB. I know two approaches to achieve this:

• Lazy loaded properties
• Separate entity class (the idea is taken from here)

Lazy loaded properties are good when you only loading data from DB. But if you have to save entities then there is a risk to lose your data if you forget to fetch lazy loaded properties beforehand.

So I chose the second approach.

I created separate small NodeEntityLite class with properties mapped to non-LOB columns of NODES table. I modified NodeEntity class so it inherits from NodeEntityLite class. I changed the mappings for my classes and used union-subclass for inheritance.

public class NodeEntityLite {
  public virtual long Id { get; set; }
  public virtual string Code { get; set; }
}
public class NodeEntity : NodeEntityLite {
  public virtual string NOTE { get; set; } // type:clob
}

FluentNHibernate mapping for NodeEntityLite class is

public void Override(AutoMapping<NodeEntityLite> mapping) {
    mapping.Table("NODES");
    mapping.UseUnionSubclassForInheritanceMapping();
}

FluentNHibernate mapping for NodeEntity class is

public void Override(AutoMapping<NodeEntity> mapping) {
    mapping.Table("NODES");
    mapping.Map(e => e.NOTE).CustomType("StringClob").CustomSqlType("NCLOB");
}

I expected that when I execute select n from NodeEntityLite n where n.Id = :p0 HQL then NHibernate generates SQL commands without NOTE column:

select nodeentity0_.ID as id1_87_, 
       nodeentity0_.CODE as code2_87_
    from from NODES nodeentity0_ 
    where nodeentity0_.ID=:p0;

But NHibernate generates absolutely different SQL command (NOTE column is not skipped as I expected):

select nodeentity0_.ID as id1_87_, 
       nodeentity0_.CODE as code2_87_, 
       nodeentity0_.NOTE as note14_87_, 
       nodeentity0_.clazz_ as clazz_ 
    from ( select ID, CODE, NOTE, 1 as clazz_ from NODES ) nodeentity0_ 
    where nodeentity0_.ID=:p0;

I tried to change inheritance and to use other mappings but without success.

The question is: Can I map several classes to the same table in NHibernate to get access to different columns?

If yes, please give an example.

---

The solution (based on the suggestions from David Osborne and mxmissile) is not to use inheritance. I use common interface implementation instead of class inheritance. The working code is below:

public interface INodeLite {
  long Id { get; set; }
  string Code { get; set; }
}

public class NodeEntityLite : INodeLite {
  public virtual long Id { get; set; }
  public virtual string Code { get; set; }
}

public class NodeEntity : INodeLite {
  public virtual long Id { get; set; }
  public virtual string Code { get; set; }
  public virtual string NOTE { get; set; } // type:clob
}
...
public void Override(AutoMapping<NodeEntityLite> mapping) {
    mapping.Table("NODES");
}
...
public void Override(AutoMapping<NodeEntity> mapping) {
    mapping.Table("NODES");
    mapping.Map(e => e.NOTE).CustomType("StringClob").CustomSqlType("NCLOB");
}

Answer 1

Regardless of the inheritance, NH can map different types to the same table. I have done it, albeit without inheritance.

You should be able to remove this line from the NodeEntityLite override and achieve it:

mapping.UseUnionSubclassForInheritanceMapping();

If this proves unsuccessful, you might need to tune the automapping further. It's definitely possible though.