EF 6，LINQ如何在内部创建包含条件参数的位置

Question

I have 4 tables

tblTraining
-----------
ID
Name

tblQuestion
----------
ID
Name
Active (true / false)
(FK) TrainingID

tblAnswer
----------
ID
Name
Active (true / false)
(FK) QuestionID

tblEmployeeTraining
-------------------------
ID
(FK) EmployeeID
(FK) TrainingID

Each Training has many Questions, and each Question has many Answers. I also have tblEmployeeTraining to store every employee that do the related training.

I wanna to get the training object by training ID, but I have conditional parameter to include the Question or EmployeeTraining.

So here I call my query:

GetTraining(dbContext, ID: 1, includeEmployeeTrainings: true, includeQuestions: false);

here my GetTraining Function:

public static TrainingCourse GetTraining(dbContext, int ID = 0,
        bool includeEmployeeTrainings = false, bool includeQuestions = false)
    {
        try
        {
            IQueryable<tblTraining> training = (
                    from tc in dbContext.tblTraining
                    select tc);

            //where 
            if (ID != 0) training = training.Where(tc => tc.ID == ID);

            //include
            var includeQuestionQuery = includeQuestions ? training.Select(tc => new
            {
                Questions = tc.tblQuestions.Where(qq => qq.IsActive == true),
                Answers = tc.tblQuestions.Select(qq => qq.tblAnswers.Where(qa => qa.IsActive == true)),
            }) : null;

            var includeEmployeeTrainingsQuery = includeEmployeeTrainings ? training.Select(tc => new
            {
                EmployeeTrainings = tc.tblEmployeeTrainings.AsEnumerable()
            }) : null;

            training = training.Select(tc => new
            {
                tc,
                includeQuestionQuery,
                includeEmployeeTrainingsQuery,
            }).AsEnumerable().Select(tc => tc.tc).AsQueryable();

            return training.FirstOrDefault();
        }
        catch
        {
            throw;
        }
    }

the above code will work perfectly if I call like this

GetTraining(dbContext, 1, includeEmployeeTrainings: true, includeQuestions: true);

but get an error if I call like this

GetTraining(dbContext, 1, includeEmployeeTrainings: false, includeQuestions: true);

Unable to create a null constant value of type 'System.Linq.IQueryable 1[[<>f__AnonymousType2 1[[System.Collections.Generic.IEnumerable`1[[.....]]'. Only entity types, enumeration types or primitive types are supported in this context

I know the error is because in GetTraining Function, the includeEmployeeTrainingsQuery is null. But how do I do solve this problem ?

Answer 1

Your problem is, that the compiler cannot work out the type of null in your anonymous type variable training .

One solution would be to create a strongly typed result using ViewModels. Those can be null using something like (Viewmodelclass)null . Or you can simply avoid assigning null using classic if statements, instead of ternary operators:

public class QuestionsQueryVm {
    public foo bar {get; set; }
}
public class EmplyeeTrainingVm {
    public foo bar {get; set; }
}


public class TrainingVm {
    public QuestionsVm  Questions { get; set;}
    public EmployeeTrainingsVm EmployeeTraining { get; set; }
    public Training Training
}

So now you can start from the top:

var training = dbContext.tblTraining.Where(tc => tc.ID == ID)
var trainingVm = new TrainingVm {
    Training = training.FirstOrDefault(), // should be only one
}; 

if(includeQuestions) 
{
   var questions = training.Select(tc => new QuestionsQueryVm
   {
        Questions = tc.tblQuestions.Where(qq => qq.IsActive == true),
        Answers = tc.tblQuestions.Select(qq => qq.tblAnswers.Where(qa => qa.IsActive == true)),
   });
   trainingVm.Questions = questions;
}

if(includeEmployeeTrainings) 
{
    employeeTraining = training.Select(tc => newEmplyoeeTraingVm
    {
         EmployeeTrainings = tc.tblEmployeeTrainings.AsEnumerable()
    });
    trainingVm.EmployeeTraining = employeeTraining;
}