如何在一个事务中向两个表添加行？

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。

我的场景很简单。我有两个三个表，Student、Courses 和 Enrollments，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向 Student 表和 Enrollment 表添加记录，我将如何去做。 假设 Course 表中有三门课程： CourseId：1，CourseName：数学，TotalCredits：4 CourseId：2，CourseName：物理，TotalCredits：4 CourseId: 3, CourseName: Chemistry, TotalCredits: 4

如果我尝试添加学生和注册，我将如何获得要添加的学生的 ID？伪代码如下：

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。而是设置与您的 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。与其创建一个新实例然后一个一个地填充属性，不如这样做：

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();

另外，如果你要添加一个新的相关项，你实际上可以结合这两个对象初始化：

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