Inheritance with EF Code First

Table per Hierarchy (TPH)

An entire class hierarchy can be mapped to a single table. This table includes columns for all properties of all classes in the hierarchy. The concrete subclass represented by a particular row is identified by the value of a type discriminator column. You don’t have to do anything special in Code First to enable TPH. It's the default inheritance mapping strategy:

public abstract class BillingDetail 
{
    public int BillingDetailId { get; set; }
    public string Owner { get; set; }        
    public string Number { get; set; }
}
 
public class BankAccount : BillingDetail
{
    public string BankName { get; set; }
    public string Swift { get; set; }
}
 
public class CreditCard : BillingDetail
{
    public int CardType { get; set; }                
    public string ExpiryMonth { get; set; }
    public string ExpiryYear { get; set; }
}
 
public class InheritanceMappingContext : DbContext
{
    public DbSet<BillingDetail> BillingDetails { get; set; }
}

Table per Type (TPT)

Table per Type is about representing inheritance relationships as relational foreign key associations. Every class/subclass that declares persistent properties—including abstract classes—has its own table. The table for subclasses contains columns only for each noninherited property (each property declared by the subclass itself) along with a primary key that is also a foreign key of the base class table. This approach is shown in the following figure:

public abstract class BillingDetail
{
    public int BillingDetailId { get; set; }
    public string Owner { get; set; }
    public string Number { get; set; }
}
 
[Table("BankAccounts")]
public class BankAccount : BillingDetail
{
    public string BankName { get; set; }
    public string Swift { get; set; }
}
 
[Table("CreditCards")]
public class CreditCard : BillingDetail
{
    public int CardType { get; set; }
    public string ExpiryMonth { get; set; }
    public string ExpiryYear { get; set; }
}
 
public class InheritanceMappingContext : DbContext
{
    public DbSet<BillingDetail> BillingDetails { get; set; }
}

Table per Concrete Type

able per Concrete type is somehow the simplest approach suggested, yet using TPC with EF is one of those concepts that has not been covered very well so far and I've seen in some resources that it was even discouraged. The reason for that is just because Entity Data Model Designer in VS2010 doesn't support TPC (even though the EF runtime does). That basically means if you are following EF's Database-First or Model-First approaches then configuring TPC requires manually writing XML in the EDMX file which is not considered to be a fun practice. Well, no more. You'll see that with Code First, creating TPC is perfectly possible with fluent API just like other strategies and you don't need to avoid TPC due to the lack of designer support as you would probably do in other EF approaches.

public abstract class BillingDetail
{
    public int BillingDetailId { get; set; }
    public string Owner { get; set; }
    public string Number { get; set; }
}
        
public class BankAccount : BillingDetail
{
    public string BankName { get; set; }
    public string Swift { get; set; }
}
        
public class CreditCard : BillingDetail
{
    public int CardType { get; set; }
    public string ExpiryMonth { get; set; }
    public string ExpiryYear { get; set; }
}
    
public class InheritanceMappingContext : DbContext
{
    public DbSet<BillingDetail> BillingDetails { get; set; }
        
    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        modelBuilder.Entity<BankAccount>().Map(m =>
        {
            m.MapInheritedProperties();
            m.ToTable("BankAccounts");
        });
 
        modelBuilder.Entity<CreditCard>().Map(m =>
        {
            m.MapInheritedProperties();
            m.ToTable("CreditCards");
        });            
    }
}

The Importance of EntityMappingConfiguration Class

As a side note, it worth mentioning that EntityMappingConfiguration class turns out to be a key type for inheritance mapping in Code First. Here is an snapshot of this class:

namespace System.Data.Entity.ModelConfiguration.Configuration.Mapping
{
    public class EntityMappingConfiguration<TEntityType> where TEntityType : class
    {
        public ValueConditionConfiguration Requires(string discriminator);
        public void ToTable(string tableName);
        public void MapInheritedProperties();
    }
}

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http://weblogs.asp.net/manavi/inheritance-mapping-strategies-with-entity-framework-code-first-ctp5-part-1-table-per-hierarchy-tph