EF Core Check Constraint Discriminator Constraints

In EF Core, TPH inheritance uses one database table to maintain data for all of the entity types in an inheritance hierarchy. It allows you to map a .NET type hierarchy to a single database table.

• A discriminator column is added to your table in this mapping pattern, which determines which entity type is represented by the particular row when reading query results from the database.
• EF will materialize different .NET types in the hierarchy based on this value.
• In the typical case, your hierarchy will have a closed set of .NET types; but as with enums, the database discriminator column can contain anything.
• If EF encounters an unknown discriminator value when reading query results, the query will fail.

The EFCore.CheckConstraints allows you to create check constraints to make sure that only valid value is stored in the discriminator column. The following is a simple model that contains inheritance.

public abstract class Person
{
    public int Id { get; set; }
    public string FullName { get; set; }
}

public class Student : Person
{
    public DateTime EnrollmentDate { get; set; }
}

public class Teacher : Person
{
    public DateTime HireDate { get; set; }
}

To activate discriminator check constraints, we need to call the UseDiscriminatorCheckConstraints() method.

public class EntityContext : DbContext
{
    protected override void OnConfiguring(DbContextOptionsBuilder opBuilder)
    {
        opBuilder
            .UseSqlServer("Data Source=(localdb)\\ProjectsV13;Initial Catalog=PeopleContextDb;")
            .UseDiscriminatorCheckConstraints();
    }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Teacher>().HasBaseType<Person>();
        modelBuilder.Entity<Student>().HasBaseType<Person>();
    }

    public DbSet<Person> People { get; set; }
}

Now let's execute your application and you will see the following table is created for you.

CREATE TABLE [dbo].[People] (
    [Id]             INT            IDENTITY (1, 1) NOT NULL,
    [FullName]       NVARCHAR (MAX) NULL,
    [Discriminator]  NVARCHAR (MAX) NOT NULL,
    [EnrollmentDate] DATETIME2 (7)  NULL,
    [HireDate]       DATETIME2 (7)  NULL,
    CONSTRAINT [PK_People] PRIMARY KEY CLUSTERED ([Id] ASC),
    CONSTRAINT [CK_People_Discriminator] CHECK ([Discriminator]=N'Teacher' OR [Discriminator]=N'Student' OR [Discriminator]=N'Person')
);