Ef核心LazyLoading - 类型集合的访问嵌套导航属性抛出DetachedLazyLoadingWarning错误

Question

I tried to access the GradeInfo property of the student's latest grade using ef core 2.1

I listed the models at the end of the question

var students = applicationDbContext.Students.Where(s => s.Id ==2)
    .Select(student => new { 
        LatestGrade = student.Grades.OrderBy(g => g.Date).FirstOrDefault(),
        Id = student.Id
        }).ToList();

In addition I use lazy loading proxies (from Microsoft.EntityFrameworkCore.Proxies) in startup.cs

services.AddDbContext<ApplicationDbContext>(options => 
    options.UseLazyLoadingProxies()
           .UseSqlServer(connectionString));

The error which is thrown is:

"Error generated for warning Microsoft.EntityFrameworkCore.Infrastructure.DetachedLazyLoadingWarning: An attempt was made to lazy-load navigation property 'Info' on detached entity of type 'GradeProxy'. Lazy-Loading is not supported for detached entities or entities that are loaded with 'AsNoTracking()'."

In addition, i want to state that I tried to add Inculde to the sudents's dbset as described in the following code, but the problem did not resolved.

var student = applicationDbContext.Students.Include( s => s.Grades )
                .ThenInclude( grade => grade.Info).Where(...).Select(...)

---

models

class Student{
   public int Id { get; set;}
   public virtual ICollection<Grade> Grades { get; set;}

   ...
}

class Grade {
   public virtual GrandeInfo Info { get; set;}
   public DateTime Date { get; set;}
}

class GrandeInfo {
  public int Id { get; set;}
  public int Score { get; set;}
}

Answer 1

For this issue, it is caused by that if you change the query so that it no longer returns instances of the entity type that the query began with, then the include operators are ignored. You could refer Ignored includes .

And, currently, it is not supported to query the navigation property by Include , refer Are .Include/.ThenInclude supposed to work with Select in RC1-final? #4641 .

For a workaround, you need to query all the columns in the db and then query the expected type at client side.

var students = _context.Students
                    .Where(s => s.Id == 2)
                    .ToList()
                    .Select(student => new
                    {
                        LatestGrade = student.Grades.OrderBy(g => g.Date).FirstOrDefault(),
                        Id = student.Id
                    })
                    .ToList();