在ASP.NET MVC中使用IQueryable仅选择特定的列

Question

I'm kinda new to asp.net mvc. I'm currently working with code that has been developed by another person and have come to a dead end.

IQueryable<DatatableTrainingHistory> trainingView = (from b in db.DatatableTrainingHistorys
                                                     select b);

/* Returns the format needed to output DataTable extension */
return DataTablesResult.Create(trainingView, dataTableParam, uv => new
{
      //stuff runs here
});

What this code does is it selects data from all columns in a table. What I currently need is for the code to select specific columns, let's say TrainingName , TrainingTime , Location , and Quota . The output must be an IQueryable type since the code uses this extension to output the data.

I have tried numerous ways to try to solve this.

(1)

IQueryable<DatatableTrainingHistory> trainingView = (from b in db.DatatableTrainingHistorys
                                                    select new DatatableTrainingHistory
                                                    {
                                                        TrainingName = b.TrainingName,
                                                        TrainingTime = b.TrainingTime,
                                                        Quota = b.Quota,
                                                        Location = b.Location,
                                                    });

The code above outputs an error:

The entity or complex type 'AppModel.DatatableTrainingHistory' cannot be constructed in a LINQ to Entities query.

(2)

IQueryable<DatatableTrainingHistory> trainingView = (from b in db.DatatableTrainingHistorys
                                                    select new
                                                    {
                                                        TrainingName = b.TrainingName,
                                                        TrainingTime = b.TrainingTime,
                                                        Quota = b.Quota,
                                                        Location = b.Location,
                                                    }).AsQueryable();

The code above shows an error saying:

Cannot implicitly convert type 'System.Linq.IQueryable<AnonymousType#1>' to 'System.Linq.IQueryable<Project.Models.DatatableTrainingHistory>'. An explicit conversion exists (are you missing a cast?)

Can anyone kindly point me to the right direction? Thank you.

Answer 1

(1). DatatableTrainingHistory is an entity object and you simply cannot use entity object in LINQ query.

(2). You're returning anonymous object , so you should use var for it

var trainingView = (from b in db.DatatableTrainingHistorys
                                                    select new
                                                    {
                                                        TrainingName = b.TrainingName,
                                                        TrainingTime = b.TrainingTime,
                                                        Quota = b.Quota,
                                                        Location = b.Location,
                                                    }).AsQueryable();

There're two possible solutions for this:

• use anonymous object (as point (2) in your question)
• introduce new data transfer object

both of them has pros and cons , the second solution requires more efforts, but it will give you strongly type => less errors. There're couple of libraries to do the object mapping automatically for you (for example: AutoMapper ), maybe you can try them out.

Cheers.