Question

我刚开始学习C＃和.NET。我的问题可能很简单，但任何帮助都会很棒。

我的情景非常简单。我有两个三个表，学生，课程和入学，定义如下。

using System;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
using System.Linq;
using System.Web;

namespace MyFirstProject.Models
{
    public class Student
    {
        [Key]
        public int StudentId { get; set; }

        [Required(ErrorMessage = "Custom error message: First Name is required")]
        [MaxLength(50)]
        [Display(Name = "First Name")]
        public string FirstName { get; set; }

        [Required(ErrorMessage = "Custom error message: Last Name is required")]
        [MaxLength(50)]
        [Display(Name = "Last Name")]
        public string LastName { get; set; }

        [DisplayFormat(DataFormatString = "{0:d}", ApplyFormatInEditMode = true)]
        public DateTime EnrollmentDate { get; set; }
        public virtual ICollection<Enrollment> Enrollments { get; set; }  
    }
}

using System;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
using System.ComponentModel.DataAnnotations.Schema;
using System.Linq;
using System.Web;

namespace MyFirstProject.Models
{
    public class Course
    {
        [DatabaseGenerated(DatabaseGeneratedOption.None)]
        public int CourseId { get; set; }

        public string CourseName { get; set; }

        [Range(0, 5, ErrorMessage = "Number of credits must be between 0 and 5")]
        public int TotalCredits { get; set; }

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

using System;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
using System.Linq;
using System.Web;

namespace MyFirstProject.Models
{
    public class Enrollment
    {
        public int EnrollmentId { get; set; }
        public int StudentId { get; set; }
        public int CourseId { get; set; }

        [DisplayFormat(DataFormatString = "{0:#.#}", ApplyFormatInEditMode = true, NullDisplayText = "No Grade")]
        public decimal? Grade { get; set; }
        //Every enrollment has at least one student and one course.
        public virtual Student student { get; set; }
        public virtual Course course { get; set; }
    }
}

现在，如果我想在一个数据库事务中将记录添加到Student表和Enrollment表中，我将如何进行此操作。我们假设课程表中有三门课程： CourseId：1，CourseName：Math，TotalCredits：4 CourseId：2，CourseName：Physics，TotalCredits：4 CourseId：3，CourseName：Chemistry，TotalCredits：4

如果我尝试添加学生和注册，我将如何获得我正在添加的学生的ID？ Psuedo代码如下：

Student s = new Student();
s.FirstName = "John";
s.LastName = "Doe";
db.Student.Add(s);
db.SaveChanges();   
Enrollment e = new Enrollment();
e.CourseId = 1;
e.StudentId = s.StudentId //I need to get the student id for the above added student. How?
db.Enrollment.Add(e);
db.SaveChanges();

Answer 1

不要手动设置ID。而是设置与student属性的关系。

e.student = s;

对于它的价值，您还应该使用对象初始化，因为它使代码更清晰。而不是创建一个新实例，然后逐个填充属性，这样做：

var s = new Student
{
    FirstName = "John",
    LastName = "Doe"
};

var e = new Enrollment
{
    CourseId = 1,
    student = s
};

// this will actually add student as well since it's related
db.Enrollment.Add(e);
db.SaveChanges();

此外，如果您要添加新的相关项，则实际上可以组合两个对象初始化：

var e = new Enrollment
{
    CourseId = 1,
    student = new Student
    {
        FirstName = "John",
        LastName = "Doe"
    }
};

如何在一个事务中将行添加到两个表？

1 个答案: