Not set "Discriminator" column when save entity in Entity Framework - sql

Question

I have a base class Person:
[KnownType(typeof(Doctor))]
public abstract class Person
{
public int PersonId { get; set; }
public string FirstName { get; set; }
public string MiddleName { get; set; }
public string LastName { get; set; }
public string Gender { get; set; }
public DateTime BirthDate { get; set; }
public string Email { get; set; }
public string MobilePhoneNumber { get; set; }
public string HomePhoneNumber { get; set; }
public bool IsGlobal { get; set; }
public bool IsDeleted { get; set; }
public bool IsApproved { get; set; }
public int? FacilityId { get; set; }
public int? AddressId { get; set; }
public virtual FacilityView Facility { get; set; }
public virtual Address Address { get; set; }
public virtual ICollection<Organization> Organizations { get; set; }
public virtual ICollection<TenantEntity> TenantEntities { get; set; }
}
And have nested class Doctor:
public class Doctor : Person
{
public string Speciality { get; set; }
}
Table Person script
CREATE TABLE [core].[Person](
[PersonId] [int] IDENTITY(1,1) NOT NULL,
[Discriminator] [nvarchar](255) NOT NULL,
[FirstName] [nvarchar](255) NOT NULL,
[MiddleName] [nvarchar](255) NULL,
[LastName] [nvarchar](20) NOT NULL,
[Gender] [nvarchar](20) NOT NULL,
[BirthDate] [date] NOT NULL,
[Email] [nvarchar](250) NULL,
[MobilePhoneNumber] [nvarchar](250) NULL,
[HomePhoneNumber] [nvarchar](250) NULL,
[IsGlobal] [bit] NOT NULL,
[IsDeleted] [bit] NOT NULL,
[IsApproved] [bit] NOT NULL,
[FacilityId] [int] NULL,
[AddressId] [int] NULL,
[Speciality] [nvarchar](250) NULL,
And when i try to save new Doctor entity i have an error:
Cannot insert the value NULL into column 'Discriminator'
What i do wrong in this situation?Why EF not save "Doctor" value in Discriminator field?
UPDATE:
part from DBContext:
public DbSet<Person> Persons { get; set; }
#region Person
modelBuilder.Entity<Person>()
.ToTable("Person", "core")
.HasKey(t => t.PersonId);
modelBuilder.Entity<Person>()
.HasOptional(t => t.Facility);
modelBuilder.Entity<Person>()
.HasOptional(t => t.Address);
modelBuilder.Entity<Person>()
.HasMany(x => x.Organizations)
.WithMany()
.Map(x =>
{
x.MapLeftKey("PersonId");
x.MapRightKey("OrganizationId");
x.ToTable("PersonOrganization", "core");
});
modelBuilder.Entity<Person>()
.HasMany(x => x.TenantEntities)
.WithMany()
.Map(x =>
{
x.MapLeftKey("PersonId");
x.MapRightKey("TenantEntityId");
x.ToTable("PersonTenantEntity", "core");
});
#endregion

Answer 1

The default inheritance mapping does not define a discriminator column if only one derived class exists.
Your code will work if you define a second class that derives from Person:
public class Nurse : Person
{
public string Whatever {get;set;}
}
Or add the discriminator explicitly to your model.
modelBuilder.Entity<Doctor>().Map(p => p.Requires("Discriminator").HasValue("Doctor"));