Keeping NHibernate from loading some fields

This is a follow on question to My earlier question on lazy loading properties. Since the application is an enhancement to a production application in a fairly major enterprise, and it currently running using NHib 1.2, upgrading to version 3 is not going to happen, so the suggested answer of using Lazy Properties in 3.0 won't work for me.

To summarize the problem, I have simple database with 2 tables. One has about a dozen simple fields, plus a one to many relation to the second table as a child table. Two of the fields are very large blobs (several megabytes each), and I want to, effectively, lazy load them. This table has about 10 records, and they populate a grid at start up, but access to the large blobs are only needed for whatever row is selected.

The object structure looks something like this:

[Serializable]
[Class(Schema = "dbo", Lazy = false)]
public class DataObject
{
    [Id(-2, Name = "Identity", Column="Id")]
    [Generator(-1, Class="native")]
    public virtual long Identity { get; set;}
    [Property]
    public string FieldA { get; set;}
    [Property]
    public byte[] LongBlob {get; set;}
    [Property]
    public string VeryLongString { get; set;}
    [Bag(-2, Cascade=CascadeStyle.All, Lazy= false, Inverse=true)]
    [Key(-1, Column="P开发者_C百科arentId")]
    [OneToMany(0, ClassType=typeof(DataObjectChild))]
    public IList<DataObjectChild> ChildObjects { get; set;}
}

Currently, the table is accessed with a simple query:

          objectList = (List<DataObject>) Session.CreateQuery("from DataObject").List<DataObject>();

And that takes care of everything, including loading the child objects.

What I would like is a query to do exactly the same thing, except select a list of the properties of the DataObject that includes everything EXCEPT the two blobs. Then I can add custom property Getters that will go out and load those properties when they are accessed.

So far, I have not been successful at doing that.

Things I have tried:

a) constructing an HQL query using a SELECT list. It looks something like this:

objectList = (List<DataObject>) Session.CreateQuery(
    "SELECT new DataObject " +
    "(obj.Identity, obj.FieldA) " +    
    "from DataObject as obj")

That works, though I have to add a constructor to the DataObject that will construct it from fields I select, which is rather cumbersome. But then it doesn't load and expand the list of child objects, and I can't figure out how to do that easily (and don't really want to - I want to tell NHib to do it.)

b) removing the [Property] attribute from the two fields in the object definition. That keeps the NHibernate.Mapping.Attributes from mapping those fields, so they don't get included in the query, but then I have no way to access them from NHib at all, including writing them out when I go to save a new or modified DataObject.

I'm thinking there has to be an easier way. Can somebody point me in the right direction?

Thanks

I think you're on the right path. However, you're going to have to do more manual work since you're using a very old version of NHibernate. I would fetch projections of your entities that contain only the columns you want to eagerly load. Then when the UI requests the large blob objects, you're going to have to write another query to get them and supply them to the UI.

Another option would be to have a second class (e.g. SmallDataObject) with the same mapping (but without the blobs) and use that for the list. When editing a list item you would load the class with the blobs using the id of the selected list item.

In any case, modifying the mapping after the creation of the SessionFactory is not possible, so you cannot get rid of the mapped properties on demand.