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2.4.4 EF Core -- 关系

• 一对多
• 一对一
• 多对多
• 示例

关系：

一对多

// Dependent Entity 主表public class Blog{    // Principal Key 标识键/可能是主键或者备用键（唯一性约束）    public int BlogId { get; set; }        public string Url { get; set; }    // Collection navigation property 关联多个从表的属性集合（集合属性）    public List
   
     Posts { get; set; }}// Principal Entity 从表public class Post{    public int PostId { get; set; }        public string Title { get; set; }        public string Content { get; set; }    // Foreign Key 外键（指向主表中的 Principal Key）    // Inverse navigation property 反向导航属性    public int BlogId { get; set; }    // Inverse navigation property 反向导航属性    public Blog Blog { get; set; }}
   

一对一

// Principal Entity 从表public class Blog{    public int BlogId { get; set; }        public string Url { get; set; }    public List
   
     Posts { get; set; }}// Dependent Entity 主表public class Post{    public int PostId { get; set; }        public string Title { get; set; }        public string Content { get; set; }    // Reference navigation property 一对一时指向另外一张表（引用属性）    public Blog Blog { get; set; }}
   

多对多

public class Post{    public int PostId { get; set; }        public string Title { get; set; }        public string Content { get; set; }    public ICollection
   
     Tags { get; set; }}public class Tag{    public string TagId { get; set; }    public ICollection
    
      Posts { get; set; }}
    
   

示例

一对多

一个 Project 对应多个 ProjectGroup

在 Project 实体中添加 ProjectGroup 列表

public List
   
     Groups { get; set; }
   

迁移

dotnet ef migrations add ProjectGroupCollectionProperty

生成集合属性 ProjectGroupCollectionProperty

protected override void Up(MigrationBuilder migrationBuilder){    migrationBuilder.AlterColumn
   
    (        name: "ProjectId",        table: "ProjectGroups",        nullable: true,        oldClrType: typeof(string),        oldType: "longtext CHARACTER SET utf8mb4",        oldNullable: true);    migrationBuilder.CreateIndex(        name: "IX_ProjectGroups_ProjectId",        table: "ProjectGroups",        column: "ProjectId");    migrationBuilder.AddForeignKey(        name: "FK_ProjectGroups_Projects_ProjectId",        table: "ProjectGroups",        column: "ProjectId",        principalTable: "Projects",        principalColumn: "Id",        onDelete: ReferentialAction.Restrict);}
   

手动配置

class MyContext : DbContext{    public DbSet
   
     Blogs { get; set; }    public DbSet
    
      Posts { get; set; }    protected override void OnModelCreating(ModelBuilder modelBuilder)    {        // 先在从表上建立一对一的关系，再从主表上建立一对多的关系        modelBuilder.Entity
     
      ()            .HasOne(p => p.Blog)            .WithMany(b => b.Posts);    }}public class Blog{    public int BlogId { get; set; }    public string Url { get; set; }    public List
      
        Posts { get; set; }}public class Post{    public int PostId { get; set; }    public string Title { get; set; }    public string Content { get; set; }    public Blog Blog { get; set; }}
      
     
    
   

LighterDbContext

// 一对一modelBuilder.Entity
   
    ().HasOne
    
     (g => g.Project);// 一对多modelBuilder.Entity
     
      ().HasOne
      
       (g => g.Project).WithMany(p => p.Groups);
      
     
    
   

多对多

为 Project 和 Subject 建立中间表 SubjectProject

public class Project : Entity{    public string Title { get; set; }    public DateTime StartDate { get; set; }    public DateTime EndDate { get; set; }    public string SupervisorId { get; set; }    public string PlanId { get; set; }    public List
   
     Groups { get; set; }        public List
    
      SubjectProjects { get; set; }}public class Subject : Entity{    public string Title { get; set; }    public string Content { get; set; }        public List
     
       SubjectProjects { get; set; }}public class SubjectProject : Entity{    public string ProjcetId { get; set; }    public Project Project { get; set; }    public string SubjectId { get; set; }    public Subject Subject { get; set; }}
     
    
   

配置多对多关系

LighterDbContext

// 多对多（两组一对多）modelBuilder.Entity
   
    ()    .HasOne
    
     (s => s.Project)    .WithMany(p => p.SubjectProjects)    .HasForeignKey(s => s.ProjcetId);    modelBuilder.Entity
     
      ()    .HasOne
      
       (s => s.Subject)    .WithMany(p => p.SubjectProjects)    .HasForeignKey(s => s.SubjectId);
      
     
    
   

迁移

dotnet ef migrations add SubjectProjectManyToManyRelation

SubjectProjectManyToManyRelation

table.ForeignKey(    name: "FK_SubjectProject_Projects_ProjcetId",    column: x => x.ProjcetId,    principalTable: "Projects",    principalColumn: "Id",    onDelete: ReferentialAction.Restrict);table.ForeignKey(    name: "FK_SubjectProject_Subject_SubjectId",    column: x => x.SubjectId,    principalTable: "Subject",    principalColumn: "Id",    onDelete: ReferentialAction.Restrict);

中间表创建了两个外键，形成多对多

EF Core 5.0 多对多实现

public class Post{    public int PostId { get; set; }    public string Title { get; set; }    public string Content { get; set; }    public ICollection
   
     Tags { get; set; }}public class Tag{    public string TagId { get; set; }    public ICollection
    
      Posts { get; set; }}
    
   

迁移的时候会自动生成中间表

联接实体类型配置 HasMany

modelBuilder    .Entity
   
    ()    .HasMany(p => p.Tags)    .WithMany(p => p.Posts)    .UsingEntity(j => j.ToTable("PostTags"));
   

GitHub源码链接：

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