如何加载导航属性？

Question

我正在尝试使用急切加载的导航属性来创建查询。 有没有办法加载导航属性的属性？ 当我尝试以下代码时，出现“序列不包含任何元素”异常。

       var viewModel = new InstructorIndexData();
        viewModel.Instructors = db.Instructors.Include(i => i.OfficeAssignment)
            .Include(i=>i.Assignments) 
             .Include(i => i.Students)
             .Include (i=>i.Enrollments )
            .Include(i => i.Courses.Select(c => c.Department))
            .OrderBy(i => i.LastName);

        viewModel.Courses = viewModel.Instructors.Where(i => i.InstructorID == id).Single().Courses;
        viewModel.Enrollments = viewModel.Courses.Where(x => x.CourseID == courseID).Where( e=>e.Enrollments.First().InstructorFullName == "Kim Abercrombie").Single().Enrollments;

但是，当我使用：

            viewModel.Enrollments = viewModel.Courses.Where(x => x.CourseID == courseID).Single().Enrollments;

它会很好地填充我的viewModel.Enrollment属性。 有没有办法在linq查询中访问这种类型的复杂属性？

Answer 1

有没有办法加载导航属性的属性？

您似乎已经知道如何渴望加载多个级别：

viewModel.Instructors = db.Instructors.Include(i => i.OfficeAssignment)
        .Include(i=>i.Assignments) 
         .Include(i => i.Students)
         .Include (i=>i.Enrollments )
        .Include(i => i.Courses.Select(c => c.Department))
        .OrderBy(i => i.LastName);

这是您可能会得到异常的地方（或者可能是对.Single()其他使用）：

viewModel.Enrollments = viewModel.Courses
   .Where(x => x.CourseID == courseID)
   .Where( e=>e.Enrollments.First().InstructorFullName == "Kim Abercrombie")
   .Single().Enrollments;

来自.Single()的文档

返回序列中的唯一元素，如果序列中不存在一个元素，则引发异常。

Answer 2

这是我解决的方法。 var selectTemp = viewModel.Courses.Where（x => x.CourseID == courseID）.Single（）。Enrollments;

            var selectedTeacher = from t in selectTemp where t.InstructorFullName == teacher select t;


            viewModel.Enrollments = selectedTeacher;