4.4关系配置

实体配置类

class CommentConfig : IEntityTypeConfiguration<Comment>
{
	public void Configure(EntityTypeBuilder<Comment> builder)
	{
		builder.ToTable("T_Comments");
        //只需要配置一个实体类就能确定两者的关系
		builder.HasOne<Article>(c => c.Article).WithMany(a => a.Comments)
			.IsRequired();
        //HasOne<Article>(c => c.Article)表示Comment类中色Article属性指向Article属性
        //WithMany(a => a.Comments)表示一个Article类对应多个Comment类，并且通过Article中的Comments属性
        //进行访问
		builder.Property(c => c.Message).IsRequired().IsUnicode();
	}
}

生成两张表如下，注意在T_Comments表中生成了ArticleId外键，指向T_Articles中的Id

Article a1 = new Article();
a1.Title = "微软发布.NET 6大版本的首个预览";
a1.Content = "微软昨日在一篇官网博客文章中宣布了 .NET 6 首个预览版本的到来。";
Comment c1 = new Comment() { Message = "支持" };
Comment c2 = new Comment() { Message = "微软太牛了" };
Comment c3 = new Comment() { Message = "火钳刘明" };
a1.Comments.Add(c1);
a1.Comments.Add(c2);
a1.Comments.Add(c3);
using TestDbContext ctx = new TestDbContext();
ctx.Articles.Add(a1);//只需要加入Article就可以，会自动增加Comment对象到数据库
await ctx.SaveChangesAsync();

关联数据的获取

Article a = ctx.Articles.Include(a => a.Comments).Single(a => a.Id == 1);

Include方法起到了关联查询作用，用它生成对其他关联实体的查询操作
使用Include不仅仅能够查询到id=1的文章，也能查询到该文章所对应的评论

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);
}

关系外键属性设置

像上面案例所示，会在“多端”的T_Comments表中自动生成外键，该外键不能在Comments中直接获取，因为Comments实体类中并没用定义外键。如果有单独获取外键的需求，可以增加一个long类型的ArticleId属性，然后再关系配置上使用HasForeignKey(c=>c.ArticleId)

class CommentConfig : IEntityTypeConfiguration<Comment>
{
	public void Configure(EntityTypeBuilder<Comment> builder)
	{
		builder.ToTable("T_Comments");
		builder.HasOne<Article>(c => c.Article).WithMany(a => a.Comments)
			.IsRequired().HasForeignKey(c => c.ArticleId);
		builder.Property(c => c.Message).IsRequired().IsUnicode();
	}
}

单导航属性

上例子中，Article类中声明了comments属性，在Commennt类中声明了Article属性，可以通过任何一方获取对方信息，这叫做双向导航。

单想导航是只在“多端”声明导航属性，不用在“一端”声明

class User
{
	public long Id { get; set; }
	public string Name { get; set; }//姓名
}
//请假实体类，里面涉及好多人
class Leave
{
	public long Id { get; set; }
	public User Requester { get; set; }//申请者
	public User? Approver { get; set; } //审批者 可能为空
	public string Remarks { get; set; } //说明
	public DateTime From { get; set; } //开始日期
	public DateTime To { get; set; } //结束日期
	public int Status { get; set; }//状态
}

配置类

class LeaveConfig : IEntityTypeConfiguration<Leave>
{
	public void Configure(EntityTypeBuilder<Leave> builder)
	{
		builder.ToTable("T_Leaves");
        //WithMany()没有传递参数，因为User类中没有对应Leave类的属性
		builder.HasOne<User>(l => l.Requester).WithMany();
		builder.HasOne<User>(l => l.Approver).WithMany();
		builder.Property(l => l.Remarks).HasMaxLength(1000).IsUnicode();
	}
}

插入数据

User u1 = new User { Name = "杨中科" };
Leave leave1 = new Leave();
leave1.Requester = u1;
leave1.From = new DateTime(2021, 8, 8);
leave1.To = new DateTime(2021, 8, 9);
leave1.Remarks = "家里三套房拆迁，回家处理";
leave1.Status = 0;
using TestDbContext ctx = new TestDbContext();
ctx.Users.Add(u1);//如果单独插入u1，则不会插入leave1，因为user中没有leave的属性
ctx.Leaves.Add(leave1);//如果单独插入leave也可以，会自动插入u1,因为有指向User的属性
await ctx.SaveChangesAsync();

一对一

一对一的关系，由于双方都是“平等”的关系，外键可以设立在任何一方，所以必须显式在一种一个实体类中声明外键属性

//案例：订单及物流信息
class OrderConfig : IEntityTypeConfiguration<Order>
{
	public void Configure(EntityTypeBuilder<Order> builder)
	{
		builder.ToTable("T_Orders");
		builder.Property(o => o.Address).IsUnicode();
		builder.Property(o => o.Name).IsUnicode();
		builder.HasOne<Delivery>(o => o.Delivery).WithOne(d => d.Order)
			.HasForeignKey<Delivery>(d => d.OrderId);//物流类（Delivery）中的OrderId作为指向Order的外键
	}
}

多对多

实体类，学生和老师

class Student
{
	public long Id { get; set; }
	public string Name { get; set; }
	public List<Teacher> Teachers { get; set; } = new List<Teacher>();
}
class Teacher
{
	public long Id { get; set; }
	public string Name { get; set; }
	public List<Student> Students { get; set; } = new List<Student>();
}

配置类，只需要在一个配置类中设定两者的关系

在一对一和一对多中，只需要在某个表中增加外键就可以，但是在多对多中，需要引入额外的数据库表来保存两张表之间的对应关系。

class StudentConfig : IEntityTypeConfiguration<Student>
{
	public void Configure(EntityTypeBuilder<Student> builder)
	{
		builder.ToTable("T_Students");
		builder.Property(s => s.Name).IsUnicode().HasMaxLength(20);
		builder.HasMany<Teacher>(s => s.Teachers).WithMany(t => t.Students)
			.UsingEntity(j => j.ToTable("T_Students_Teachers"));//使用UsingEntity来配置中间表T_Students_Teachers
	}
}

插入和查询

Student s1 = new Student { Name = "tom" };
Student s2 = new Student { Name = "lily" };
Student s3 = new Student { Name = "lucy" };
Student s4 = new Student { Name = "tim" };
Student s5 = new Student { Name = "lina" };
Teacher t1 = new Teacher { Name = "杨中科" };
Teacher t2 = new Teacher { Name = "罗翔" };
Teacher t3 = new Teacher { Name = "刘晓艳" };
t1.Students.Add(s1);
t1.Students.Add(s2);
t1.Students.Add(s3);
t2.Students.Add(s1);
t2.Students.Add(s3);
t2.Students.Add(s5);
t3.Students.Add(s2);
t3.Students.Add(s4);

using TestDbContext ctx = new TestDbContext();
ctx.AddRange(t1, t2, t3);//是对Add的简化调用，仍然是一个一个加入
ctx.AddRange(s1, s2, s3, s4, s5);
await ctx.SaveChangesAsync();
using Microsoft.EntityFrameworkCore;

using TestDbContext ctx = new TestDbContext();
foreach (var t in ctx.Teachers.Include(t => t.Students))
{
	Console.WriteLine($"老师{t.Name}");
	foreach (var s in t.Students)
	{
		Console.WriteLine($"---{s.Name}");
	}
}