EF 7:如何以一对多关系加载相关实体

Question

我有以下代码.为什么我的导航属性(课程中的要求和要求中的课程)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

首先,您不需要两次配置您的关系,您只需要执行一次:

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

第二件事是如果你正在进行查询并且你期望延迟加载相关属性,我恐怕是不可能的.EF 7还不支持延迟加载.如您所见,有一个积压项目跟踪延迟加载.因此,如果您需要加载相关实体,则应使用显式加载Include方法:

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