我记得在EF 导航属性应为virtual：

public class Blog 
{  
    public int BlogId { get; set; }  
    public string Name { get; set; }  
    public string Url { get; set; }  
    public string Tags { get; set; }

    public virtual ICollection<Post> Posts { get; set; }  
}

但是我看的是EF Core，但并不认为它是虚拟的：

public class Student
    {
        public int ID { get; set; }
        public string LastName { get; set; }
        public string FirstMidName { get; set; }
        public DateTime EnrollmentDate { get; set; }

        public ICollection<Enrollment> Enrollments { get; set; }
    }

不再需要了吗？