通过连接表在实体框架中表达一对多关系

Question

In database design, a common practice is to express a one-to-many relationship with a foreign key on the "one" (or child) side. Entity framework handles this situation well.

However, I have the situation where a one-to-many relationship is expressed via a join table in which one of the two foreign keys on the table has a unique constraint.

How can Entity Framework be configured to utilize this join table?

In my current state, when doing a simple read query on the one/child entity, Entity Frameworks is throwing an exception---as expected---that the one/child table is missing a column by the conventional name based on the navigation property.

Answer 1

What you are looking for is optional one-to-many association with a join table . The motivation behind this is that we always try to avoid nullable columns in a relational database schema. Information that is unknown degrades the quality of the data you store. Therefore, an optional entity association, be it one-to-one or one-to-many, is best represented in an SQL database with a join table to avoid nullable foreign key columns.

All that being said, EF unfortunately does not support this type of mapping. If you really want to implement this then you might want to take a look at other ORM Frameworks that support one-to-many association with a join table like NHibernate .

Answer 2

If I have the below DDL.

Employee (M:N) to ParkingArea. However, a constraint keeps only one Employee in the link table, thus a 1:N.

-- START 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'

Where the constraint "CONST_UNIQUE_EmpUUID" is the setup of which you are speaking.

EmployeeEntity like this:

[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);
}

You would map like this:

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);

    }
}

ParkingArea like this:

[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);
        }
    }