Cannot map LINQ to Entities - asp.net-mvc

Question

I'm not very clear with writing linq queries. I write a query to select only certain columns from a table using linq lambda expression and I get the error that linq cannot be constructed to entities. The same query when I write using linq to select all columns I don't get any errors and I get all the columns, which i later filter out in the view. But I want to use the lambda to select only certain columns.
Code snippet:
ViewModel:
public class StaggingInternalCashExceptionViewModel
{
public OutputCash OutputCash { get; set; }
public IEnumerable<StaggingInternalException> StaggingInternalException { get; set; }
//list of results of Stagginginternalcashexception
}
Controller:
public ActionResult Exceptionstest(string dd1, string dd2, string dd3)
{
StaggingInternalExceptionViewModel _app = new StaggingInternalExceptionViewModel();
_app.StaggingInternalException = db2.StaggingInternalExceptions.Where(x => x.Level1 == dd1 && x.Level2 == dd2 ).Select(i => new StaggingInternalException
{
StaggingInternalRowID = i.StaggingInternalRowID,
Category = i.Category,
EnterText1 = i.EnterText1,
InternalAmount = i.InternalAmount,
ExternalAmount = i.ExternalAmount
});
_app.StaggingInternalException = (from p in db2.StaggingInternalExceptions
where p.LoadID==loadid && p.Level1 == dd1 && p.Level2 == dd2 select p);
}
In the above code, the lambda expression throws an error when I'm trying to select only certain columns from the table or if we are speaking in terms of entity classes, only certain properties. But the query returns all the columns. Should I be using DTOS? I'm not sure what the use of data transfer objects is. Some explanation on this would be great. Thanks.

Answer 1

You need to use a DTO.
A dto is just an object that you map your result to. In your case it would be
public class StaggingInternalExceptionViewModel
{
public int StaggingInternalRowID { get; set; }
public int Category { get; set; }
... //rest of properties
}
You need to change your StaggingInternalCashExceptionViewModel to use the StaggingInternalException DTO
public class StaggingInternalCashExceptionViewModel
{
public OutputCash OutputCash { get; set; }
public IEnumerable<StaggingInternalExceptionViewModel> StaggingInternalException { get; set; }
//list of results of Stagginginternalcashexception
}
Then your expression stays the basically the same but you select a new StaggingInternalExceptionViewModel instead of StaggingInternalException
StaggingInternalExceptionViewModel _app = new StaggingInternalCashExceptionViewModel();
_app.StaggingInternalException = db2.StaggingInternalExceptions.Where(x => x.Level1 == dd1 && x.Level2 == dd2 ).Select(i => new StaggingInternalExceptionViewModel
{
StaggingInternalRowID = i.StaggingInternalRowID,
Category = i.Category,
EnterText1 = i.EnterText1,
InternalAmount = i.InternalAmount,
ExternalAmount = i.ExternalAmount
});

Answer 2

Linq to Entities doesn't let you project a query using an entity type because you can end up losing information at loading an entity partially and trying later to save that entity to your DB. So, you must project your queries when you need partial information of an entity whether using a DTO or an anonymous type.
If you need to use the entity type, then don't project using Select method, the only thing is you're going to load all the properties, but I think this is not the case because you don't need all the data ;).