如何将记录首先插入到许多代码Approch中 [英] How to Insert Records into many to many code first Approch

问题描述

我的实体类

My entity class

public class person
    {
        public int PersonId { get; set; }
        public string FirstName { get; set; }
        public string LastName { get; set; }

        public virtual ICollection<Course> courses { get; set;}


       
    }

    public class Course
    {
        public int CourseId { get; set; }
        public string Title { get; set; }


        public virtual ICollection<person> Persons { get; set; }

       

    }

我的上下文类代码

My Context class code

public class Context : DbContext
    {
        public Context()
            : base("test")
        { }
        public DbSet<person> Persons { get; set; }
        public DbSet<Course> Courses { get; set; }

 modelBuilder.Entity<Course>()
                .HasMany(c => c.Persons)
                .WithMany(p => p.courses)
                .Map(m =>
                {
                    m.MapLeftKey("CourseId");
                    m.MapRightKey("PersonId");
                    m.ToTable("PersonCourses");


                }
                );

当我尝试将对象添加到人员类的课程集合时

i得到错误，如此对象引用未设置为对象的实例。 br />


和我的代码在这里：

When I try add Objects to course Collection which is in persons class
i am getting error like this Object reference not set to an instance of an object.

and My Code is here:

using (Context db = new Context())
{
Course stoptalk = new Course();
Course ruby = new Course();
Course rails = new Course();
 
stoptalk.Title = "Stoptalk";
ruby.Title = "Ruby";
rails.Title = "Rails";
 

db.Courses.Add(stoptalk);
db.Courses.Add(ruby);
db.Courses.Add(rails);
 
person john = new person();
john.FirstName = "John";
john.LastName = "Meclian";
 

if (john.courses == null)
{
john.courses.Add(stoptalk);
john.courses.Add(rails);
db.Persons.Add(john);
db.SaveChanges();
}
 
}

推荐答案

这部分代码会给你这个问题

This part of the code will give you this problem

if (john.courses == null)
{
    john.courses.Add(stoptalk);
    john.courses.Add(rails);
    db.Persons.Add(john);
    db.SaveChanges();
}

因为如果 john.courses == null 为真，那么添加操作无法执行，因为没有创建 john.courses 的实例。



您需要做的是正确初始化会员课程。



示例：

because if john.courses == null is true, the Add operation cannot be executed as no instance of john.courses has been created.

What you need to do is to initialize the member courses properly.

Example:

public class person
{
    public int PersonId { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }

    public virtual ICollection<Course> courses { get; set;}

    // Default constructor
    public person()
    {
        PersonId = 0;
        FirstName = "";
        LastName = "";
        courses = new List<Course>;
    }
}
</course>

然后更改if语句：

Then change the if-statement:

if (john.courses.Count == 0)
{
  ...
}

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