In Defense of Lazy Loading

1.cs

public class Student : Entity
{
    public virtual string Name { get; set; }
    public virtual decimal TotalDebt { get; set; }

    public virtual IList<Enrollment> Enrollments { get; set; }
    public virtual IList<SportsActivity> SportsActivities { get; set; }
}

public class Enrollment : Entity
{
    public virtual Student Student { get; set; }
    public virtual Course Course { get; set; }
    public virtual Grade Grade { get; set; }
}

public enum Grade
{
    A = 1, B = 2, C = 3, D = 4, F = 5
}

public class Course : Entity
{
    public virtual string Name { get; set; }
    public virtual int Credits { get; set; }
}

public class SportsActivity : Entity
{
    public virtual Sports Sports { get; set; }
    public virtual DateTime PlayingSince { get; set; }
}

public class Sports : Entity
{
    public virtual string Name { get; set; }
}

2.cs

public IActionResult Disenroll(long studentId, int enrollmentNumber)
{
    Student student = _studentRepository.GetById(studentId);
    Enrollment enrollment = student.Enrollments[enrollmentNumber];
    student.Enrollments.Remove(enrollment);

    return Ok();
}

3.sql

-- Roundtrip 1
SELECT *
FROM dbo.Student s
WHERE s.StudentID = @StudentID

-- Roundtrip 2
SELECT *
FROM dbo.Enrollment e
WHERE e.StudentID = @StudentID

4.sql

-- Roundtrip 1
SELECT *
FROM dbo.Student s
INNER JOIN dbo.Enrollment e ON s.StudentID = e.StudentID
INNER JOIN dbo.Course c ON e.CourseID = c.CourseID
WHERE s.StudentID = @StudentID

SELECT *
FROM dbo.SportsActivity a
INNER JOIN dbo.Sports s ON a.SportsID = s.SportsID
WHERE a.StudentID = @StudentID

5.cs

public IActionResult Enroll(long studentId, string sportsName)
{
    Sports sports = _sportsRepository.GetByName(sportsName);
    Student student = _studentRepository.GetById(studentId);

    if (student.TotalDebt > 10000M)
        return Error($"Cannot enroll into {sportsName}, too much debt");

    student.SportsActivities.Add(new SportsActivity
    {
        PlayingSince = DateTime.Now,
        Sports = sports
    });

    return Ok();
}

6.cs

public sealed class StudentRepository
{
    public Student GetByIdWithEnrollments(long studentId)
    {
        /* Load only enrollments */
    }

    public Student GetByIdWithSportActivities(long studentId)
    {
        /* Load only sport activities */
    }

    public Student GetByIdWithEnrollmentsAndSportActivities(long studentId)
    {
        /* Load everything */
    }
}