literal or constant as part of composite key in EF code first

Question

I am relatively new to the Code First approach to Entity Framework. I have used the Database First approach for a while now, but the Code First seems to be a better fit for the application I am currently developing. I am working with an existing MS SQL database, and I am not allowed to make any changes whatsoever to the database. The reason why I am using Code First is because the Fluent API allows me to dynamically assign a table name to a class.

That said, I have a predicament where I need to assign a relationship between 2 tables. One table, ArCodes, has a composite key made up of the CodeType and the Code (both are strings). The CodeType column determins the type of code and the Code column is the identifier unique to the code type.

public class ArCode {

    [Column("cod_typ", Order = 0), Key]
    public string CodeType { get; set; }

    [Column("ar_cod", Order = 1), Key]
    public string Code { get; set; }

    [Column("desc")]
    public string Description { get; set; }

}

The other table, Invoices, needs to have a relationship to the ArCodes table for both a "ship via" code and a "terms" code.

public class Invoice {
    [Column("pi_hist_hdr_invc_no"), Key]
    public int InvoiceNumber { get; set; }

    [Column("shp_via_cod")]
    public string ShipViaCode { get; set; }

    public ArCode ShipVia { get; set; }

    [Column("terms_cod")]
    public string TermsCode { get; set; }

    public ArCode Terms { get; set; }

}

I would like to setup the relationship for both the "ShipVia" property and the "Terms" property. However, I am not sure how to do so in regards to the CodeType portion of the composite key. For "ship via" codes the Code Type should be "S", and code "terms" codes, the code type should be "T".

I have tried the following in by DB Context, but it did not work:

    protected override void OnModelCreating(DbModelBuilder modelBuilder) {
        // setup the table names
        modelBuilder.Entity<ArCode>().ToTable("ARCODS" + CompanyCode);
        modelBuilder.Entity<Invoice>().ToTable("IHSHDR" + CompanyCode);

        //
        // setup the relationships
        //

        // 1 Invoice <--> 0-1 Ship Via AR Codes
        modelBuilder.Entity<Invoice>()
            .HasOptional(invoice => invoice.ShipVia)
            .WithMany()
            .HasForeignKey(invoice => new { TheType = "S", invoice.ShipViaCode })
            ;

        base.OnModelCreating(modelBuilder);
    }

Any help would be appreciated.

Update #1

Ok, I reduced my code to its simplest form, and I followed the solution as provided by @GertArnold.

public abstract class ArCode {

    [Column("cod_typ")]
    public string CodeType { get; set; }

    [Column("ar_cod")]
    public string Code { get; set; }

    [Column("terms_desc")]
    public string TermsDescription { get; set; }

    [Column("terms_typ")]
    public string TermsType { get; set; }

    [Column("shp_via_desc")]
    public string ShipViaDescription { get; set; }

    [Column("tax_desc")]
    public string TaxDescription { get; set; }

}

public class TermsCode : ArCode { }
public class ShipViaCode : ArCode { }

public class Invoice {
    [Column("pi_hist_hdr_invc_no"), Key]
    public int InvoiceNumber { get; set; }

    [Column("hdr_invc_dat")]
    public DateTime InvoiceDate { get; set; }

    [Column("shp_via_cod")]
    public string ShipViaCode { get; set; }

    public ShipViaCode ShipVia { get; set; }

    [Column("terms_cod")]
    public string TermsCode { get; set; }

    public TermsCode Terms { get; set; }

    public Invoice() {
    }

}

public class PbsContext : DbContext {

    public DbSet<Invoice> Invoices { get; set; }

    protected override void OnModelCreating(DbModelBuilder modelBuilder) {

        modelBuilder.Entity<Invoice>().ToTable("IHSHDR");

        modelBuilder.Entity<ArCode>().HasKey(r => r.Code).ToTable("ARCODS");

        modelBuilder.Entity<TermsCode>().Map(m => m.Requires("CodeType")
               .HasValue("T").HasColumnType("varchar").HasMaxLength(1).IsRequired())
               .ToTable("ARCODS");
        modelBuilder.Entity<ShipViaCode>().Map(m => m.Requires("CodeType")
               .HasValue("S").HasColumnType("varchar").HasMaxLength(1).IsRequired())
               .ToTable("ARCODS");

        base.OnModelCreating(modelBuilder);
    }

    public PbsContext()
        : base("name=PbsDatabase") {
    }
}

However, the following code returns an error:

PbsContext context = new PbsContext();
var invoice = context.Invoices.OrderByDescending(r => r.InvoiceDate).FirstOrDefault();

error 3032: Problem in mapping fragments starting at line 28:Condition member 'ArCode.cod_typ' with a condition other than 'IsNull=False' is mapped. Either remove the condition on ArCode.cod_typ or remove it from the mapping.

If I remove the "CodeType" column from the ArCode class and change all "CodeType" references to the database column name of "cod_typ" within the OnModelCreating event, then the statement above executes without error. However, invoice.ShipVia and invoice.Terms will both be null event though there is a matching record in the database.

Update #2

public abstract class ArCode {

    [Column("ar_cod")]
    public string Code { get; set; }

    [Column("terms_desc")]
    public string TermsDescription { get; set; }

    [Column("terms_typ")]
    public string TermsType { get; set; }

    [Column("shp_via_desc")]
    public string ShipViaDescription { get; set; }

    [Column("tax_desc")]
    public string TaxDescription { get; set; }

}

public class TermsCode : ArCode { }
public class ShipViaCode : ArCode { }

public class Invoice {
    [Column("pi_hist_hdr_invc_no"), Key]
    public int InvoiceNumber { get; set; }

    [Column("hdr_invc_dat")]
    public DateTime InvoiceDate { get; set; }

    [Column("shp_via_cod")]
    public ShipViaCode ShipVia { get; set; }

    [Column("terms_cod")]
    public TermsCode Terms { get; set; }

    public Invoice() {
    }

}

public class PbsContext : DbContext {

    public DbSet<Invoice> Invoices { get; set; }

    protected override void OnModelCreating(DbModelBuilder modelBuilder) {

        modelBuilder.Entity<Invoice>().ToTable("IHSHDR");

        modelBuilder.Entity<ArCode>().HasKey(r => r.Code).ToTable("ARCODS");

        modelBuilder.Entity<TermsCode>().Map(m => m.Requires("CodeType")
               .HasValue("T").HasColumnType("varchar").HasMaxLength(1).IsRequired())
               .ToTable("ARCODS");
        modelBuilder.Entity<ShipViaCode>().Map(m => m.Requires("CodeType")
               .HasValue("S").HasColumnType("varchar").HasMaxLength(1).IsRequired())
               .ToTable("ARCODS");

        base.OnModelCreating(modelBuilder);
    }

    public PbsContext()
        : base("name=PbsDatabase") {
    }
}

Now, the following code returns an error:

PbsContext context = new PbsContext();
var invoice = context.Invoices.OrderByDescending(r => r.InvoiceDate).FirstOrDefault();

EntityCommandExecutionException - Invalid column name 'ShipVia_Code'. Invalid column name 'Terms_Code'.

Answer 1

What you want is impossible for EF. ArCode has a composite key, so any association to it will have to use two Properties. That means that in Invoice you'd need four properties (two pairs) to refer to the two ArCode objects. But two of these properties (those for CodeType) are not backed up by columns in the database, so EF can not map them.

But... there is a way that may help you out. You could create two derived classes from ArCode and let Invoice refer to those by single-property associations. But then you have to divert from the model as such and fool EF a bit by defining a single key:

public abstract class ArCode { ... } // abstract!

public class TermsCode : ArCode { }

public class ShipViaCode : ArCode { }

protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
    modelBuilder.Entity<Invoice>().ToTable("IHSHDR");

    modelBuilder.Entity<Invoice>().HasOptional(i => i.Terms).WithOptionalDependent().Map(m => m.MapKey("terms_cod"));
    modelBuilder.Entity<Invoice>().HasOptional(i => i.ShipVia).WithOptionalDependent().Map(m => m.MapKey("shp_via_cod"));

    modelBuilder.Entity<ArCode>().HasKey(a => a.Code).ToTable("ARCODS");
    modelBuilder.Entity<TermsCode>().Map(m => m.Requires("CodeType")
        .HasValue("T").HasColumnType("varchar").HasMaxLength(1).IsRequired())
        .ToTable("ARCODS");
    modelBuilder.Entity<ShipViaCode>().Map(m => m.Requires("CodeType")
        .HasValue("S").HasColumnType("varchar").HasMaxLength(1).IsRequired())
        .ToTable("ARCODS");

    base.OnModelCreating(modelBuilder);
}

public class Invoice
{
    [Column("pi_hist_hdr_invc_no"), Key]
    public int InvoiceNumber { get; set; }

    public ShipViaCode ShipVia { get; set; }

    public TermsCode Terms { get; set; }
}

This may work for you if you don't have to insert ARCODS records through EF. It won't allow you to insert records with identical Codes, although the database would allow it. But I expect the content of ARCODS to be pretty stable and maybe it is enough to fill it with a script.