在 LINQ 中创建数据库时查询数据库中的父参考答案

【问题标题】：Parent Reference in Query DB when Creating database in LINQ在 LINQ 中创建数据库时查询数据库中的父参考
【发布时间】：2021-11-11 11:52:42
【问题描述】：

我有以下课程

public class Children {
     public Children (){} // constructor
     public int Id {get;set;}, 
     public string ChildName {get; set;}
}
public class Parents {
     public Parents(){} // constructor
     public int Id {get;set;}, 
     public string ParentName {get;set;}
     public Children Child {get; set;}
}

我想要每个父母 1 个孩子，在我的配置中

  public void Configure(EntityTypeBuilder<Parents> configuration)
  {  
       configuration.ToTable("Parents");
       configuration.Property(p => p.Id).HasField("_id");
       configuration.HasOne(a => a.ChildClass).WithMany().OnDelete(DeleteBehavior.Cascade);
  }


  public void Configure(EntityTypeBuilder<Children> configuration)
  {  
       configuration.ToTable("Children");
       configuration.Property(p => p.Id).HasField("_id");
  }

当我使用实体框架 5 构建器时，我会从数据库中返回 2 个表

CREATE TABLE [dbo].[Parents](
     [Id] [int] IDENTITY(1,1) NOT NULL,
     [ChildId] [int] NOT NULL,
     [ParentName] [nvarchar](max) NULL,
CONSTRAINT [PK_Parents] PRIMARY KEY CLUSTERED 
(
    [Id] ASC
)
WITH 
    (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, 
    ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
    ) ON [PRIMARY] TEXTIMAGE_ON [PRIMARY]

ALTER TABLE [dbo].[Parents]  WITH CHECK ADD  CONSTRAINT 
    [FK_Parents_Children_Child] FOREIGN KEY([ChildId])
    REFERENCES [dbo].[Children] ([Id])
    ON DELETE CASCADE
GO

ALTER TABLE [dbo].[Parents] CHECK CONSTRAINT [FK_Parents_Parents_Children_ChildId]
GO

CREATE TABLE [dbo].[Children](
    [Id] [int] IDENTITY(1,1) NOT NULL,
    [ChildName] [nvarchar](max) NULL,
    [ParentId] [int] NULL,
CONSTRAINT [PK_Children] PRIMARY KEY CLUSTERED 
(
     [Id] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, 
      ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY] TEXTIMAGE_ON [PRIMARY]
GO
 ALTER TABLE [dbo].[Children]  WITH CHECK ADD  CONSTRAINT [FK_Children_Parent_ParentId] FOREIGN 
     KEY([ParentId])
     REFERENCES [dbo].[Parents] ([Id])
GO

ALTER TABLE [dbo].[Children] CHECK CONSTRAINT [FK_Children_Parent_ParentId]
GO

基本上我陷入了一个循环，因为我无法插入 Parents 记录，并且我无法插入儿子，因为他们需要两组 Id，我不知道为什么 Children 表需要引用 @987654326 @表，如果父表已经引用了Children表。

【问题讨论】：

标签： c# sql-server asp.net-core entity-framework-core

【解决方案1】：

根据您的流利映射，您似乎已经建立了一对多的关系，其中每个孩子可以有多个父母。但你实际上想要一个one-to-one relationship。

public class Children {
    public Children (){}
    public int Id { get;set; }
    public string ChildName { get; set; }
    public int ParentId { get; set; }
    public Parents Parent { get; set; }
}
public class Parents {
     public Parents(){}
     public int Id { get; set; }
     public string ParentName { get; set; }
     public Children Child { get; set; }
}

public void Configure(EntityTypeBuilder<Parents> configuration)
{  
    configuration.ToTable("Parents");
    configuration.Property(p => p.Id).HasField("_id");
    configuration.HasOne(p => p.Child).WithOne(c => c.Parent).HasForeignKey<Children>(c => c.ParentId).OnDelete(DeleteBehavior.Cascade);
}

但是，生成的 SQL 似乎与您的模型和流畅的配置都不匹配。

【讨论】：