通过连接表在实体框架中表达一对多关系

Question

在数据库设计中，一种常见的做法是在“一”（或子）端用外键表示一对多关系。实体框架很好地处理了这种情况。

但是，我遇到了通过连接表表示一对多关系的情况，其中表上的两个外键之一具有唯一约束。

如何配置实体框架以使用此连接表？

在我目前的状态下，当对一个/子实体执行简单的读取查询时，Entity Frameworks 抛出异常——正如预期的那样——一个/子表缺少基于常规名称的列在导航属性上。

Answer 1

如果我有以下 DDL。

员工 (M:N) 到 ParkingArea。但是，一个约束在链接表中只保留一个 Employee，因此是 1:N。

-- 启动 TSQL

Use OrganizationReverseDB
GO


SET NOCOUNT ON


IF EXISTS (SELECT * FROM dbo.sysobjects WHERE id = object_id(N'[dbo].[EmployeeParkingAreaLink]') and OBJECTPROPERTY(id, N'IsUserTable') = 1) 
BEGIN DROP TABLE [dbo].[EmployeeParkingAreaLink] 
END 
GO

IF EXISTS (SELECT * FROM dbo.sysobjects WHERE id = object_id(N'[dbo].[ParkingArea]') and OBJECTPROPERTY(id, N'IsUserTable') = 1) 
BEGIN 
DROP TABLE [dbo].[ParkingArea] 
END 
GO

IF EXISTS (SELECT * FROM dbo.sysobjects WHERE id = object_id(N'[dbo].[Employee]') and OBJECTPROPERTY(id, N'IsUserTable') = 1) 
BEGIN 
DROP TABLE [dbo].[Employee] 
END 


IF EXISTS (SELECT * FROM dbo.sysobjects WHERE id = object_id(N'[dbo].[Employee]') and OBJECTPROPERTY(id, N'IsUserTable') = 1) 
BEGIN DROP TABLE [dbo].[Employee] 
END 


IF EXISTS (SELECT * FROM dbo.sysobjects WHERE id = object_id(N'[dbo].[Department]') and OBJECTPROPERTY(id, N'IsUserTable') = 1) 
BEGIN DROP TABLE [dbo].[Department] 
END 

GO


CREATE TABLE [dbo].[Department](
    [DepartmentUUID] [uniqueidentifier] NOT NULL,
    [TheVersionProperty] [timestamp] NOT NULL,
    [DepartmentName] [nvarchar](80) NULL,
    [CreateDate] [datetime] NOT NULL,
    )


ALTER TABLE dbo.[Department] ADD CONSTRAINT PK_Department PRIMARY KEY NONCLUSTERED ([DepartmentUUID]) 
GO

ALTER TABLE [dbo].[Department] ADD CONSTRAINT CK_DepartmentName_Unique UNIQUE ([DepartmentName]) 
GO


CREATE TABLE [dbo].[Employee] ( 

    [EmployeeUUID] [uniqueidentifier] NOT NULL,
    [ParentDepartmentUUID] [uniqueidentifier] NOT NULL,
    [TheVersionProperty] [timestamp] NOT NULL,
    [SSN] [nvarchar](11) NOT NULL,
    [LastName] [varchar](64) NOT NULL,
    [FirstName] [varchar](64) NOT NULL,
    [CreateDate] [datetime] NOT NULL,
    [HireDate] [datetime] NOT NULL
    )

GO

ALTER TABLE dbo.Employee ADD CONSTRAINT PK_Employee PRIMARY KEY NONCLUSTERED (EmployeeUUID) 
GO


ALTER TABLE [dbo].[Employee] ADD CONSTRAINT CK_SSN_Unique UNIQUE (SSN) 
GO

ALTER TABLE [dbo].[Employee] ADD CONSTRAINT FK_EmployeeToDepartment FOREIGN KEY (ParentDepartmentUUID) REFERENCES dbo.Department (DepartmentUUID) 
GO





IF EXISTS (SELECT * FROM dbo.sysobjects WHERE id = object_id(N'[dbo].[ParkingArea]') and OBJECTPROPERTY(id, N'IsUserTable') = 1) 
BEGIN DROP TABLE [dbo].[ParkingArea] 
END 
GO

CREATE TABLE [dbo].[ParkingArea] 
( 
ParkingAreaUUID [UNIQUEIDENTIFIER] NOT NULL DEFAULT NEWSEQUENTIALID() 
, ParkingAreaName varchar(24) not null 
, CreateDate smalldatetime not null
)

GO

ALTER TABLE dbo.ParkingArea ADD CONSTRAINT PK_ParkingArea PRIMARY KEY NONCLUSTERED (ParkingAreaUUID) 
GO

ALTER TABLE [dbo].[ParkingArea] ADD CONSTRAINT CK_ParkingAreaName_Unique UNIQUE (ParkingAreaName) 
GO



IF EXISTS (SELECT * FROM dbo.sysobjects WHERE id = object_id(N'[dbo].[EmployeeParkingAreaLink]') and OBJECTPROPERTY(id, N'IsUserTable') = 1) 
BEGIN DROP TABLE [dbo].[EmployeeParkingAreaLink] 
END 
GO

CREATE TABLE [dbo].[EmployeeParkingAreaLink] ( 
    /* [LinkSurrogateUUID] [uniqueidentifier] NOT NULL, */
    [TheEmployeeUUID] [uniqueidentifier] NOT NULL,
    [TheParkingAreaUUID] [uniqueidentifier] NOT NULL
)


GO

/*
ALTER TABLE dbo.EmployeeParkingAreaLink ADD CONSTRAINT PK_EmployeeParkingAreaLink PRIMARY KEY NONCLUSTERED (LinkSurrogateUUID) 
*/
GO

ALTER TABLE [dbo].[EmployeeParkingAreaLink] ADD CONSTRAINT FK_EmployeeParkingAreaLinkToEmployee FOREIGN KEY (TheEmployeeUUID) REFERENCES dbo.Employee (EmployeeUUID) 
GO

ALTER TABLE [dbo].[EmployeeParkingAreaLink] ADD CONSTRAINT FK_EmployeeParkingAreaLinkToParkingArea FOREIGN KEY (TheParkingAreaUUID) REFERENCES dbo.ParkingArea (ParkingAreaUUID) 
GO

ALTER TABLE [dbo].[EmployeeParkingAreaLink] ADD CONSTRAINT CONST_UNIQUE_EmpUUID UNIQUE (TheEmployeeUUID ) 
GO

ALTER TABLE [dbo].[EmployeeParkingAreaLink] ADD CONSTRAINT CONST_UNIQUE_EmpUUID_PAUUID UNIQUE (TheEmployeeUUID , TheParkingAreaUUID) 
GO


Insert into dbo.Department ( [DepartmentUUID], [DepartmentName] , CreateDate )
    select '10000000-0000-0000-0000-000000000001' , 'DepartmentOne' , CURRENT_TIMESTAMP
    union all   select '10000000-0000-0000-0000-000000000002' , 'DepartmentTwo' , CURRENT_TIMESTAMP

Insert into dbo.Employee ( EmployeeUUID, SSN , CreateDate, HireDate , LastName, FirstName , ParentDepartmentUUID)
    select '20000000-0000-0000-0000-000000000001' , '111-11-1111' , CURRENT_TIMESTAMP , '01/31/2001' , 'Smith' , 'John' , '10000000-0000-0000-0000-000000000001'
    union all   select '20000000-0000-0000-0000-000000000002' , '222-22-2222' , CURRENT_TIMESTAMP, '02/28/2002' , 'Jones' , 'Mary' , '10000000-0000-0000-0000-000000000002'

Insert into dbo.ParkingArea ( [ParkingAreaUUID], [ParkingAreaName] , CreateDate )
    select '30000000-0000-0000-0000-000000000001' , 'ParkingAreaOne' , CURRENT_TIMESTAMP
    union all   select '30000000-0000-0000-0000-000000000002' , 'ParkingAreaTwo' , CURRENT_TIMESTAMP


INSERT INTO [dbo].[EmployeeParkingAreaLink] (   [TheEmployeeUUID] , [TheParkingAreaUUID] )
        Select '20000000-0000-0000-0000-000000000001' , '30000000-0000-0000-0000-000000000001'
union all       Select '20000000-0000-0000-0000-000000000002' , '30000000-0000-0000-0000-000000000002'

约束“CONST_UNIQUE_EmpUUID”是您所说的设置。

EmployeeEntity 像这样：

[Serializable]
public partial class EmployeeEFEntity
{

public EmployeeEFEntity()
{
    CommonConstructor();
}
private void CommonConstructor()
{
    //this.MyParkingAreas = new List<ParkingAreaEFEntity>();
}



//EF Tweaks
public virtual Guid? ParentDepartmentUUID { get; set; }

public virtual Guid? EmployeeUUID { get; set; }

public virtual byte[] TheVersionProperty { get; set; }

public virtual DepartmentEFEntity ParentDepartment { get; set; }

public virtual string SSN { get; set; }
public virtual string LastName { get; set; }
public virtual string FirstName { get; set; }
public virtual DateTime CreateDate { get; set; }
public virtual DateTime HireDate { get; set; }

public virtual ICollection<ParkingAreaEFEntity> MyParkingAreas { get; set; }

public ParkingAreaEFEntity MyOneParkingAreaEFEntity {

    get 
    {
        return MyParkingAreas.FirstOrDefault();
    }
    set
    {
        /* check for more than one here */
        this.AddParkingArea(pa);
    }
}


public virtual void AddParkingArea(ParkingAreaEFEntity pa)
{
    if (!pa.MyEmployees.Contains(this))
    {
        pa.MyEmployees.Add(this);
    }
    if (!this.MyParkingAreas.Contains(pa))
    {
        this.MyParkingAreas.Add(pa);
    }
}


public virtual void RemoveParkingArea(ParkingAreaEFEntity pa)
{
    if (pa.MyEmployees.Contains(this))
    {
        pa.MyEmployees.Remove(this);
    }
    if (this.MyParkingAreas.Contains(pa))
    {
        this.MyParkingAreas.Remove(pa);
    }
}


public override string ToString()
{
    return string.Format("{0}:{1},{2}", this.SSN, this.LastName, this.FirstName);
}

你会这样映射：

public class EmployeeMap : EntityTypeConfiguration<EmployeeEFEntity>
{
    public EmployeeMap()
    {
        // Primary Key
        this.HasKey(t => t.EmployeeUUID);

        this.Property(t => t.EmployeeUUID).HasDatabaseGeneratedOption(DatabaseGeneratedOption.Identity);

        // Properties
        this.Property(t => t.TheVersionProperty)
            .IsRequired()
            .IsFixedLength()
            .HasMaxLength(8)
            .IsRowVersion();

        this.Property(t => t.SSN)
            .IsRequired()
            .HasMaxLength(11);

        this.Property(t => t.LastName)
            .IsRequired()
            .HasMaxLength(64);

        this.Property(t => t.FirstName)
            .IsRequired()
            .HasMaxLength(64);

        // Table & Column Mappings
        this.ToTable("Employee");
        this.Property(t => t.EmployeeUUID).HasColumnName("EmployeeUUID");
        this.Property(t => t.ParentDepartmentUUID).HasColumnName("ParentDepartmentUUID");
        this.Property(t => t.TheVersionProperty).HasColumnName("TheVersionProperty");
        this.Property(t => t.SSN).HasColumnName("SSN");
        this.Property(t => t.LastName).HasColumnName("LastName");
        this.Property(t => t.FirstName).HasColumnName("FirstName");
        this.Property(t => t.CreateDate).HasColumnName("CreateDate");
        this.Property(t => t.HireDate).HasColumnName("HireDate");

        // Relationships
        this.HasMany(t => t.MyParkingAreas)
            .WithMany(t => t.MyEmployees)
            .Map(m =>
            {
                m.ToTable("EmployeeParkingAreaLink");
                m.MapLeftKey("TheEmployeeUUID");
                m.MapRightKey("TheParkingAreaUUID");
            });

        this.HasRequired(t => t.ParentDepartment)
            .WithMany(t => t.Employees)
            .HasForeignKey(d => d.ParentDepartmentUUID);

    }
}

这样的停车区：

[Serializable]
public partial class ParkingAreaEFEntity
{

    public ParkingAreaEFEntity()
    {
        CommonConstructor();
    }
    private void CommonConstructor()
    {
        //this.MyEmployees = new List<EmployeeEFEntity>();
    }

    public virtual Guid ParkingAreaUUID { get; set; }


    public virtual string ParkingAreaName { get; set; }
    public virtual DateTime CreateDate { get; set; }

    public virtual ICollection<EmployeeEFEntity> MyEmployees { get; set; }

    public virtual void AddEmployee(EmployeeEFEntity emp)
    {
        if (!emp.MyParkingAreas.Contains(this))
        {
            emp.MyParkingAreas.Add(this);
        }
        if (!this.MyEmployees.Contains(emp))
        {
            this.MyEmployees.Add(emp);
        }
    }

    public virtual void RemoveEmployee(EmployeeEFEntity emp)
    {
        if (emp.MyParkingAreas.Contains(this))
        {
            emp.MyParkingAreas.Remove(this);
        }
        if (this.MyEmployees.Contains(emp))
        {
            this.MyEmployees.Remove(emp);
        }
    }

Answer 2

您正在寻找的是与连接表的可选一对多关联。这背后的动机是我们总是尽量避免关系数据库模式中的可空列。未知信息会降低您存储的数据的质量。因此，可选的实体关联，无论是一对一还是一对多，最好在带有连接表的 SQL 数据库中表示，以避免可为空的外键列。

话虽如此，但不幸的是，EF不支持这种类型的映射。如果您真的想实现这一点，那么您可能想看看其他支持一对多关联的 ORM 框架与连接表（如 NHibernate）。