c# - EF 7 : How to load related entities in a One-to-many relationship

Question

我有以下代码。为什么我的导航属性(Requirement in Course 和 Courses in Requirement)为空？

    public class Course : AbsEntity {
            [Key]
            public string Title { get; set; }
            public string Term { get; set; }
            public int Year { get; set; }
            public string CourseId { get; set; }        
            public double GradePercent { get; set; }        
            public string GradeLetter { get; set; }     
            public string Status { get; set; }
            public int ReqId { get; set; }

            public Requirement Requirement { get; set; }
        }

    public class Requirement : AbsEntity {
            [Key]
            public int ReqId { get; set; }
            public string ReqName { get; set; }

            public ICollection<Course> Courses { get; set; }
        }

// In DbContext
    protected override void OnModelCreating(ModelBuilder modelBuilder)
            {
                modelBuilder.Entity<Course>().HasOne(c => c.Requirement).WithMany(r => r.Courses).HasForeignKey(c => c.ReqId);
                modelBuilder.Entity<Requirement>().HasMany(r => r.Courses).WithOne(c => c.Requirement);
            }

Answer 1

第一件事是你不需要配置你的关系两次，你只需要做一次:

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
   modelBuilder.Entity<Course>().HasOne(c => c.Requirement)
                                .WithMany(r => r.Courses)
                                .HasForeignKey(c => c.ReqId);          
}

第二件事是，如果您正在执行查询并且希望延迟加载相关属性，恐怕这是不可能的。 EF 7 不支持 lazy loading然而。如您所见，有一个 backlog item跟踪延迟加载。因此，如果您需要加载相关实体，您应该使用 Include 方法显式加载:

 var query= ctx.Courses.Include(c=>c.Requirement).Where(...)...;