Getting properties from a child

Question

I have the following entities:

public class Parent
{
     int Id { get; set; }
     string ParentName { get; set; }
     List<Child> Children { get; set; }
}

public class Child
{
     int Id { get; set; }
     string ChildName { get; set; }
}

and the following dto:

public class ParentDTO
{
     int Id { get; set; }
     List<string> ChildrenNames { get; set; }
}

using QueryOver code below I can get the Parent values

 ParentDTO result = null;
 Parent parentAlias = null;
 Child childAlias = null;

 var query = session.QueryOver(() => parentAlias)              
            .JoinAlias(() => parentAlias.Children, () => childAlias, JoinType.LeftOuterJoin)                           
            .SelectList(list => list.Select(c => c.Id).WithAlias(() => result.Id)
                                    .Select(c => c.ParentName).WithAlias(() => result.Name)
//this part does not work
 .Select(c => c.Children .Select(v => v.ChildName)).WithAlias(() => result.ChildrenNames) 
//                              
)
.TransformUsing(Transformers.AliasToBean<ParentDTO>());

return query.List<ParentDTO>();

However I cant seem to be able to project the list of childName values into my ChildrenNames collection.

Any ideas?

This is not supported. Projections could go to references defined as `many-to-one` ... but not to `one-to-many` *(well without some very smart custom transformer)*, check this for more details about the **issue**: http://stackoverflow.com/questions/24517403/ and this for some workaround: http://stackoverflow.com/questions/17279307/ — Radim Köhler, Aug 13 '14 at 17:27
Easiest just to write two queries here: One to grab the parents and another to get children for each parent — Andrew Whitaker, Aug 13 '14 at 18:53

score 0 · Accepted Answer · answered Aug 13 '14 at 19:10

As some guys said in comments, you need to do two queries. Using linq, you could try something like this:

// get the parent Ids
var parentIds = session.Query<Parent>().Select(c => c.Id).ToList(); 

// get the childNames
var childNames = session.Query<Child>()
                        .Where(x => parentIds.Contains(x.ParentId)) // get on the child from parents query
                        .Select(x => new {x.Name, x.ParentId}) // get only the properties you need
                        .ToList(); // list of anon objects

// loop in memory between parentIds filling the corresponding childNames
var result = parentIds.Select(parentId => new ParentDTO() 
             { 
                Id = parentId,
                ChildrenNames = childNames.Where(x => x.ParentId == parentId).ToList()
             }).ToList();

I am not sure if it works, but you could try this in a single query:

var query = from p in session.Query<Parent>()
            let names = p.Children.Select(c => c.ChildName).ToList()
            select new ParentDTO()
            {
               Id = o.Id,
               ChildrenNames = names
            };

return query.Tolist();

Obs: I did not test it.

Getting properties from a child

1 Answers1