如何在LINQ Asp C＃中使用分组方式

Question

Reverse Engineering some reports that contain bad data. Reports written in Asp.Net C# using Linq

I can write the query in the DB, but having trouble translating to Linq.

public StudentBehaviourActivityDTO GetStudentBehaviourActivity(
            Guid userId)
        {
            List<DateTime> lastUpdatedDates = Uow.ActivityTrackingUnitValues
                .Where(atuv => atuv.Last_Updated_On != null
                    && atuv.TBH_Activity_Tracking_Unit
                        .StudentBehaviour
                        .Student
                        .TeamMembers
                        .Any(tm => tm.Id == userId))
                        .Select(atuv => (DateTime)DbFunctions
                            .TruncateTime(atuv.Last_Updated_On))
                        .ToList();

What is this loading in the toList? I assume this is loading my table: ActivityTrackingUnitValues

But then it also does then it also looks like it's joining these tables

.StudentBehaviour
.Student
.TeamMembers

I want to take the max(value) for Last_Updated_On and then group by student

Any help or explanation is greatly appreciated.

Answer 1

public StudentBehaviourActivityDTO GetStudentBehaviourActivity(
        Guid userId)
    {
        List<DateTime> lastUpdatedDates = Uow.ActivityTrackingUnitValues
            .Where(atuv => atuv.Last_Updated_On != null // check to make sure it's not null
                && atuv.TBH_Activity_Tracking_Unit
                    .StudentBehaviour //this references a class (generally a join, but it doesn't join!)
                    .Student //this references a class (generally a join, but it doesn't join!)
                    .TeamMembers //this references a class (generally a join, but it doesn't join!)
                    .Any(tm => tm.Id == userId)) // Checks to make sure it isn't null or has a count greater than 0
                    .Select(atuv => (DateTime)DbFunctions
                        .TruncateTime(atuv.Last_Updated_On)) // Returns the truncatedtime and nowthing else
                    .ToList(); // converts it to a list

From what I can gather, it's making sure the class has members available.

Answer 2

What is this loading in the toList?

Basically it seems it loads the last updated dates of the subjected user if he is in a team and updated ...

.StudentBehaviour
.Student
.TeamMembers

Those are navigational properties mainly translated into joins to get the desired data with relation to the table in hand.

so in details it does the following:

1. Gets the list of Activity Tracking Unit Values where:
   • The activity tracking unit value is updated
   • The student we are checking is a team member with the student whose behaviour is tracked by the unit responsible for this activity tracking unit value
2. Selects the day where those values are last updated

I want to take the max(value) for Last_Updated_On and then group by student

compose the base dataset:

.Select(atuv => new 
{ 
    LastUpdatedDate = (DateTime)DbFunctions.TruncateTime(atuv.Last_Updated_On)),
    StudentId = atuv.TBH_Activity_Tracking_Unit.StudentBehaviour.Student.Id
})

group by student id:

.GroupBy(sd => sd.StudentId)

select student and it max last updated date:

.Select(sd => new { StudentId = sd.Key, MaxDate = sd.Max(d => d.LastUpdatedDate) });

finally it will look something like this:

var maxUpdatedDatePerStudentList = Uow.ActivityTrackingUnitValues
                    .Where(atuv => atuv.Last_Updated_On != null
                                && atuv.TBH_Activity_Tracking_Unit
                                       .StudentBehaviour
                                       .Student
                                       .TeamMembers
                                       .Any(tm => tm.Id == userId))
                    .Select(atuv => new 
                    { 
                        LastUpdatedDate = (DateTime)DbFunctions.TruncateTime(atuv.Last_Updated_On)),
                        StudentId = atuv.TBH_Activity_Tracking_Unit.StudentBehaviour.Student.Id
                    })
                    .GroupBy(sd => sd.StudentId)
                    .Select(sd => new { StudentId = sd.Key, MaxDate = sd.Max(d => d.LastUpdatedDate) })
                    .ToList();

and you can access its data like this:

maxUpdatedDatePerStudentList.ForEach(g => Console.WriteLine($"Student {g.StudentId} max updated date is {g.MaxDate}"));