TPH Simple Config / Discriminator issue with subclasses

Question

I am using Code first EF 6.1 with DotConnect for oracle data provider and TPH. I have created one class which has 2 subclasses. I am trying to add a record into existing database table. But it throws a DbUpdateException when I try to savechanges.

Exception: {"ORA-06550: line 4, column 298:\nPL/SQL: ORA-00904: \"Discriminator\": invalid identifier\nORA-06550: line 4, column 1:\nPL/SQL: SQL Statement ignored"}

Here is my code:

Person.cs :-----------------

public class Person
{
  public Person()
  {
   Departments= new HashSet<Department>();
  }

  [Key]
  public decimal PersonId { get; set; }

  public decimal OfficeId { get; set; }    

  public decimal PersonTypeId { get; set; }


  public virtual Office Office{ get; set; }

  public virtual ICollection<Department> Departments{ get; set; }

  protected bool success = false;

  public virtual bool FindDepartment(int personType)
  {
    return success;
  }

  public class PersonHR: Person
  {
    public override bool FindDepartment(int personType)
    {
      PersonEntityModel dbcontext = new PersonEntityModel ();

      Person p = new Person ();
      p.OfficeId= 765;
      p.PersonTypeId = personType;

     try
     {
       dbcontext.Persons.Add(p);
       dbcontext.SaveChanges();

       decimal personId= p.PersonId;                
     }
     catch (DbEntityValidationException dbEx)
     {          
     }

     return success;//this is just a test method so don't worry about return.
   }
  }

  public class PersonIT : Person
  {
    public override bool FindDepartment(int personType)
    {
      return success;
    }
  }
}

PersonEntityModel.cs -------------------

public partial class PersonEntityModel: DbContext
{
  public PersonEntityModel()
   : base("name=PersonEntityModel")
  {
  }

  public virtual DbSet<Office> Offices{ get; set; }
  public virtual DbSet<Person> Persons{ get; set; }
  public virtual DbSet<Department> Departments{ get; set; }

  protected override void OnModelCreating(DbModelBuilder modelBuilder)
  {
    modelBuilder.Configurations.Add(new PersonMap());     
  }        
}  

public class PersonMap: EntityTypeConfiguration<Person>
{
  public PersonMap()
  {
    this.Property(p =>   p.PersonId).HasDatabaseGeneratedOption(DatabaseGeneratedOption.Identity);
    this.HasMany(p => p.Departments).WithRequired(d=> d.Person).HasForeignKey(d=> d.PersonId);

    //table & column mappings
    this.ToTable("TABLEPERSON");
    this.Property(p => p.PersonId).HasColumnName("PERSONID");
    this.Property(p => p.OfficeId).HasColumnName("OFFICEID");
    this.Property(p => p.PersonTypeId).HasColumnName("PERSONTYPEID");      

   //this.Map<DomainObjectModel.ObjectModel.Person.PersonHR>(m => m.Requires("TYPE").HasValue(11));
   //this.Map<DomainObjectModel.ObjectModel.Person.PersonIT>(m => m.Requires("TYPE").HasValue(22));     

  }
}

Without last 2 lines, it throws an exception:{"ORA-06550: line 4, column 298:\nPL/SQL: ORA-00904: \"Discriminator\": invalid identifier\nORA-06550: line 4, column 1:\nPL/SQL: SQL Statement ignored"}

With last 2 lines, it throws this exception: {"ORA-06550: line 4, column 298:\nPL/SQL: ORA-00904: \"TYPE\": invalid identifier\nORA-06550: line 4, column 1:\nPL/SQL: SQL Statement ignored"}

I am new to TPH. How to resolve above issues?

Also, please send me some good article to read about TPH. I have read this http://weblogs.asp.net/manavi/inheritance-mapping-strategies-with-entity-framework-code-first-ctp5-part-1-table-per-hierarchy-tph.

Appreciate your help,