nhibernate and DDD suggestion

Question

Hello,

I am fairly new to nHibernate and DDD, so please bear with me.

I have a requirement to create a new report from my SQL table. The report is read-only and will be bound to a GridView control in an ASP.NET application.

The report contains the following fields Style, Color, Size, LAQty, MTLQty, Status.

I have the entities for Style, Color and Size, which I use in other asp.net pages. I use them via repositories. I am not sure If should use the same entities for my report or not. If I use them, where I am supposed to map the Qty and Status fields?

If I should not use the same entities, should I create a new class for the report?

As said I am new to this and just trying to learn and code properly.

Thank you

Answer 1

For reports its usually easier to use plain values or special DTO's. Of course you can query for the entity that references all the information, but to put it into the list (eg. using databinding) it's handier to have a single class that contains all the values plain.

To get more specific solutions as the few bellow you need to tell us a little about your domain model. How does the class model look like?

generally, you have at least three options to get "plain" values form the database using NHibernate.

Write HQL that returns an array of values

For instance:

select e1.Style, e1.Color, e1.Size, e2.LAQty, e2.MTLQty 
from entity1 inner join entity2
where (some condition)

the result will be a list of object[]. Every item in the list is a row, every item in the object[] is a column. This is quite like sql, but on a higher level (you describe the query on entity level) and is database independent.

Or you create a DTO (data transfer object) only to hold one row of the result:

select new ReportDto(e1.Style, e1.Color, e1.Size, e2.LAQty, e2.MTLQty)
from entity1 inner join entity2
where (some condition)

ReportDto need to implement a constructor that has all this arguments. The result is a list of ReportDto.

Or you use CriteriaAPI (recommended)

session.CreateCriteria(typeof(Entity1), "e1")
  .CreateCriteria(typeof(Entity2), "e2")
  .Add( /* some condition */ )
  .Add(Projections.Property("e1.Style", "Style"))
  .Add(Projections.Property("e1.Color", "Color"))
  .Add(Projections.Property("e1.Size", "Size"))
  .Add(Projections.Property("e2.LAQty", "LAQty"))
  .Add(Projections.Property("e2.MTLQty", "MTLQty"))
  .SetResultTransformer(AliasToBean(typeof(ReportDto)))
  .List<ReportDto>();

The ReportDto needs to have a proeprty with the name of each alias "Style", "Color" etc. The output is a list of ReportDto.

Answer 2

I'm not schooled in DDD exactly, but I've always modeled my nouns as types and I'm surprised the report itself is an entity. DDD or not, I wouldn't do that, rather I'd have my reports reflect the results of a query, in which quantity is presumably count(*) or sum(lineItem.quantity) and status is also calculated (perhaps, in the page).

You haven't described your domain, but there is a clue on those column headings that you may be doing a pivot over the data to create LAQty, MTLQty which you'll find hard to do in nHibernate as its designed for OLTP and does not even do UNION last I checked. That said, there is nothing wrong with abusing HQL (Hibernate Query Language) for doing lightweight reporting, as long as you understand you are abusing it.

I see Stefan has done a grand job of describing the syntax for that, so I'll stop there :-)