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C# Entity Framework Basics LINQ Queries Writing a Detail Query

How to use the Include operator to eagerly load the course's related teacher entity?

How do I eagerly load the courses's related teachers entity in this task properly? I've looked at similar questions, but none of the suggestions work for me. I must be doing something wrong...?

Course.cs
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;

namespace Treehouse.CodeChallenges
{
    public class Course
    {
        public Course()
        {
            Students = new List<CourseStudent>();
        }

        public int Id { get; set; }
        public int TeacherId { get; set; }
        [Required, StringLength(200)]
        public string Title { get; set; }
        public string Description { get; set; }
        public int Length { get; set; }

        public Teacher Teacher { get; set; }
        public ICollection<CourseStudent> Students { get; set; }
    }
}
Repository.cs
using System.Collections.Generic;
using System.Data.Entity;
using System.Linq;

namespace Treehouse.CodeChallenges
{
    public static class Repository
    {
        public static List<Course> GetCourses()
        {
            using (var context = new Context())
            {
                return context.Courses
                              .OrderBy(c => c.Teacher.LastName)
                              .ThenBy(c => c.Teacher.FirstName)
                              .ToList();
            }
        }

        public static List<Course> GetCoursesByTeacher(string lastName)
        {
            using (var context = new Context())
            {
                return context.Courses
                              .Where(c => c.Teacher.LastName == lastName)
                              .ToList();
            }
        }

        public static Course GetCourse(int id)
        {
            using (var context = new Context())
            {
                return context.Courses
                              .Where(c => c.Id == id)
                              .Include(c => c.Teacher.Select(t => t.Teacher))
                              .SingleOrDefault();
            }
        }
    }
}

2 Answers

Steven Parker
Steven Parker
229,981 Points

Here's two hints:

  • you don't need a Where because SingleOrDefault can apply the condition to filter the results
  • you can simply reference the Teacher entity in the Include, you don' t need to select anything from it

That worked, thanks again!