NHibernate LINQ projection using helper class

Question

I'm trying to add projection to an NHibernate LINQ query (via .select()), but since I have a bit of logic I want to use a helper class rather than returning a projected model directly.

My code looks like this (shortened):

var query = Session.Query<MessageInstance>();
... (a few .Fetch and .ThenFetch calls)
var result = query.Where(specification.IsSatisfiedBy())
                  .OrderBy(m => m.CreationDate)
                  .Select(m => _messageModelHelper.BuildMessageModel(m));

_messageModelHelper is an object which transforms my MessageInstance object into a MessageModel and it contains logic to handle different scenarios, since it all depends on related entities.

This fails with error:

Field Dal.Repositories.MessageRepository._messageModelHelper' is not defined for type 'NHibernate.Linq.NhQueryable`1 [Domain.Entities.Message]'

If I avoid using a field (which is necessary for DI), and do this:

.Select(m => new MessageModelHelper().BuildMessageModel(m))

I simply get a System.NotSupportedException.

It appears my approach won't work. What can I do, while avoiding the need to return new MessageModel directly? Also I really need to keep projection, I can't operate on an enumerable.

Answer 1

You have to call the BuildMessageModel() "locally":

var result = query.Where(specification.IsSatisfiedBy())
                  .OrderBy(m => m.CreationDate)
                  .AsEnumerable() // From this point onward the query
                                  // will be executed C#-side
                  .Select(m => _messageModelHelper.BuildMessageModel(m));

Note that, as written, this won't do an SQL projection, so the full object will be selected from the DB.