How to write this linq query with Criteria or QueryOver API

Question

Is it possible to convert this code at below, written by using Query(linq) api to Criteria or QueryOver API in NHibernate? I'm using this to format data into DTO's also it works with just one round-trip to db.

Note: I tried transformers.aliastobean but I can only use one transformer at a time. Is it possible to use multiple transformer in one query?

     from entityType in Provider.GetSession().Query<crmEntityType>()
     .Fetch(x => x.Association)
     .Fetch(x => x.Fields)
     .AsEnumerable()
     where instanceIDs.Contains(entityType.Instance.instanceID)
     select new EntityTypeDTO()
     {
     ID = entityType.ID,
     Title = entityType.Title,
     Association = entityType.Association.Distinct().Select(asc => asc.ID).ToArray<int>(),
     Fields = entityType.Fields.Distinct().Select(fi => new CustomFieldDTO { 
 ID = fi.ID,
 Name =  fi.Name,
 Value = fi.Value,
 EntityType = fi.EntityType.ID,
 Type = fi.Type 
}).ToList()
     }).ToList();

Answer 1

Let's start with the QueryOver syntax:

// external filter data
instanceIDs = new int[] { 1, 2, 3 };

// aliasing
EntityTypeDTO entityDTO = null;
CustomFieldDTO fieldDTO = null;
Field field = null;

IQueryOver<EntityType, Field> query = Session.QueryOver<EntityType>()

  // filter Entity by ID's list
  .Where(Restrictions.On<EntityType>(c => c.ID).IsIn(instanceIDs))

  // Join Fields
  .JoinQueryOver<Field>(c => c.Fields, () => field)
  .SelectList(list => list

    // entity
    .Select(c => c.ID)
    .Select(c => c.Title)
    // ... more Entity properties

    // field collection
    .Select(() => field.ID)
    .Select(() => field.Name)
    // ... more Field properties
  )
  .TransformUsing(new MyTransformer()); // see below

var dtos = query.List<EntityTypeDTO>();

This QueryOver will generate the SQL statement which will contain all EntityTypes with their Fields. Now we have to extract the unique EntityType instances and fill their Fields lists

There is an overview of DTO classes (as well as QueryOver above, these contain only ver few properties as an example):

public class EntityTypeDTO
{
  public virtual int ID { get; set; }
  public virtual string Title { get; set; }
  public virtual IList<CustomFieldDTO> Fields { get; set; }
  ...
}
public class CustomFieldDTO 
{
  public virtual int ID { get; set; }
  public virtual string Name { get; set; }
  ...
}

And finally the trick MyTransformer():

public class MyTransformer : IResultTransformer
{
  // rows iterator
  public object TransformTuple(object[] tuple, string[] aliases)
  {
    var entity = new EntityTypeDTO
    {
      ID = (int)tuple[0],         // aliases should be used
      Title = tuple[1] as string  // first two are belong to Entity
    };
    var field = new CustomFieldDTO
    {
      ID = (int)tuple[2],         // last 2 columns are for a Field
      Name = tuple[3] as string   // see SelectList in QueryOver
    };
    entity.Fields = new List<CustomFieldDTO> { field };
    return entity;
  }

  // convert to DISTINCT list with populated Fields
  public System.Collections.IList TransformList(System.Collections.IList collection)
  {
    var results = new List<EntityTypeDTO>();
    foreach(var item in collection)
    {
      var entity = item as EntityTypeDTO;

      // was already the same ID appended
      var existing = results.SingleOrDefault(c => c.ID.Equals(entity.ID));
      if(existing != null)
      {
        // extend fields
        existing.Fields.Add(entity.Fields.First());
        continue;
      }

      // new ID found
      results.Add(entity);
    }
    // DISTINCT list of Entities, with populated FIELDS
    return results;
  }
  ...

MyTransformer is ad hoc one, only for this purpose... but this approach could be extended