fetch single property in object from database

Question

I am working on NET MVC 3.0 and Nhibernate 3.0. I want to fetch only one property from database to an object.

For instance, suppose I have a class Module. I want to select all the names from module table (like select modulename from module query) and an prepare a list of module objects which have only name. All other properties can be null.

I tried this using QueryOver API:

IQueryOver<ProjectModule> module = session.QueryOver<ProjectModule>()
    .Select(a=>a.Name)                   
    .TransformUsing(
        NHibernate.Transform.Transformers.AliasToBean<ProjectModule>());

pm = module.List<ProjectModule>();

pm is IList<ProjectModule> type.

Transaction gets committed successfully. No error occurred, but I get a list of module objects with all properties = null. Module name null, module id null etc.

I checked what query is executing on SQL using NUnit and got this: SELECT this_Name as y0_ FROM ProjectModule this_.

diadiora · Accepted Answer · 2012-10-04 11:19:45Z

1

To be more accurate create a DTO object, assume ProjectModuleDto, that will contain only the Name. It's not a good practice to use the the domain object with uninitialized values through your code, cause it creates confusions of filled data in various scenarious.

And the fllowing code will do the trick - populate the list of DTOs: ProjectModuleDto with correct Name of project module from database:

ProjectModuleDto projectModuleDto = null;           
IQueryOver<ProjectModule> module = session.QueryOver<ProjectModule>()
    .SelectList(
        list=>list.Select(a => a.Name).WithAlias(() => projectModuleDto.Name)
    )                 
    TransformUsing(NHibernate.Transform.Transformers.AliasToBean<ProjectModuleDto>());

pm = module.List<ProjectModuleDto>();

answered Oct 4, 2012 at 11:19

diadiora

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3 Comments

Amogh Over a year ago

this works :) thanks. but as i am an student can you please tell me few thinks 1.Why we have to use SelectList(list=>list...) Why not just select(a=>a.Name)? As i am trying to do 2. Why we have uesed '()' in WithAlias what is meaning of that?

diadiora Over a year ago

if you want to use only (a=>a.Name) than you'll have to go with the answer that is provided by Miroslav Popovic, but in this case you'll get a list of strings, and not a list of types objects, that also may contain multiple properties. Assume your ProjectModuleDto has Name and Color, than you just write .SelectList( list=>list.Select(a => a.Name).WithAlias(() => projectModuleDto.Name).Select(a => a.Color).WithAlias(() => projectModuleDto.Color) ) and you are done.

Amogh Over a year ago

Thnaks its clear to me now. what about 2. Why we have uesed '()' in WithAlias what is meaning of that?

Miroslav Popovic · Accepted Answer · 2012-10-04 09:23:32Z

1

If you are fetching only a single property, you don't need to use transformers. Try to use a List<string> directly:

var moduleNames = session.QueryOver<ProjectModule>()
    .Select(a => a.Name)                   
    .List<string>();

Read more about QueryOver syntax on NHibernate blog.

answered Oct 4, 2012 at 9:23

Miroslav Popovic

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3 Comments

Amogh Over a year ago

i don't want result in string list.i want in IList<ProjectModule>.

Miroslav Popovic Over a year ago

It's fairly easy to initialize a list of ProjectModules from a list of string, although I would rather go with diadiora's suggestion and create a separate DTO for that.

Amogh Over a year ago

Thanks for reply it's also correct solution but as i need result in ProjectModule object so i go with diadiora's Answer.Thnak you Very Much.

Alex · Accepted Answer · 2012-10-04 23:24:56Z

0

Is this what you're looking for?

  List<ProjectModule> result = new List<ProjectModule>();

  session.QueryOver<ProjectModule>()
  .Select(a => a.Name)                   
  .ToList().ForEach(delegate(string mName)
  {
       result.Add(ProjectModule() { Name = mName });
  });

answered Oct 4, 2012 at 23:24

Alex

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1 Comment

Amogh Over a year ago

Hey did you seen diadiora's solution which one is better urs or diadiora's?

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