EF core learning notes: one to many relationship configuration

Recently migrated projects . It is found that the table structure is a one to many relationship . The positioning application form corresponds to multiple mold making . I am not familiar with this relationship , Dare not change . Now fill in the foundation .

Relationships between database tables ： one-on-one 、 One to many 、 Many to many （ Need a third-party form to record ）.

stay EF core in ：

1. Relationship Attribute in entity class ;

2. FluentAPI Relationship configuration ;

3. Use relational operations .

1、 Migration generates database tables .

2、 Write code to test data insertion .

3、 There is no need to be explicit Comment Object's Article Attribute assignment （ There will be no error in the current assignment ）, There is no need to explicitly create new Comment Objects of type are added to DbContext in .EF Core Meeting “ Climb along the pole ”.

Article a1 = new Article();
a1.Title = " Microsoft released .NET 6 The first preview of the big version ";
a1.Content = " tiny xxxx";
Comment c1 = new Comment() { Message=" Support "};
Comment c2 = new Comment() { Message = " Microsoft is awesome " };
Comment c3 = new Comment() { Message = " Leave a message/comment before it becomes a top/hot tweet " };
a1.Comments.Add(c1);
a1.Comments.Add(c2);
a1.Comments.Add(c3);
using (TestDbContext ctx = new TestDbContext())
{
	ctx.Articles.Add(a1);
    // ctx.Comments.Add(c1)   There is no need to operate deliberately comment  Watch 
	await ctx.SaveChangesAsync();
}

Personal understanding ：

One to many configuration , You just need to operate article surface , Instead of deliberately operating comment surface .

let me put it another way ： Just operate parent object .EF core It will help you operate automatically Sub object

Get relational data Use   Include( )  Method

Article a = ctx.Articles.Include(a=>a.Comments).Single(a=>a.Id==1);
Console.WriteLine(a.Title);
foreach(Comment c in a.Comments)
{
	Console.WriteLine(c.Id+":"+c.Message);
}

// Include It's defined in Microsoft.EntityFrameworkCore In the namespace .
//  Take a look at the generated SQL sentence