Table-per-type (TPT) mapping

Table-per-type inheritance uses a separate table in the database to maintain data for non-inherited properties and key properties for each type in the inheritance hierarchy.

  • Table per Type is about representing inheritance relationships as relational foreign key associations.

  • Every class and subclass including abstract classes has its own table.

  • The table for subclasses contains columns only for each non-inherited property along with a primary key that is also a foreign key of the base class table.

By default, EF Core maps an inheritance hierarchy of .NET types to a single database table. This is known as table-per-hierarchy (TPH) mapping. EF Core 5.0 also allows mapping each .NET type in an inheritance hierarchy to a different database table; known as table-per-type (TPT) mapping.

Let's consider the following simple model with a mapped hierarchy.

public 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; }
}

Here is the context class implementation without any additional configuration.

public class EntityContext : DbContext
{
protected override void OnConfiguring(DbContextOptionsBuilder opBuilder)
{
opBuilder.UseSqlServer("Data Source=(localdb)\\ProjectsV13;Initial Catalog=PeopleContextDb;");
}
public DbSet<Person> People { get; set; }
public DbSet<Student> Students { get; set; }
public DbSet<Teacher> Teachers { get; set; }
}

By default, EF Core will map this to a single table.

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)
);

In the TPT mapping pattern, all the types are mapped to individual tables. Properties that belong solely to a base type or derived type are stored in a table that maps to that type.

Entity types can be mapped to different tables using mapping attributes.

[Table("People")]
public class Person
{
public int Id { get; set; }
public string FullName { get; set; }
}
[Table("Students")]
public class Student : Person
{
public DateTime EnrollmentDate { get; set; }
}
[Table("Teachers")]
public class Teacher : Person
{
public DateTime HireDate { get; set; }
}

However, mapping each entity type to a different table will instead result in one table per type.

CREATE TABLE [dbo].[People] (
[Id] INT IDENTITY (1, 1) NOT NULL,
[FullName] NVARCHAR (MAX) NULL,
CONSTRAINT [PK_People] PRIMARY KEY CLUSTERED ([Id] ASC)
);
CREATE TABLE [dbo].[Students] (
[Id] INT NOT NULL,
[EnrollmentDate] DATETIME2 (7) NOT NULL,
CONSTRAINT [PK_Students] PRIMARY KEY CLUSTERED ([Id] ASC),
CONSTRAINT [FK_Students_People_Id] FOREIGN KEY ([Id]) REFERENCES [dbo].[People] ([Id])
);
CREATE TABLE [dbo].[Teachers] (
[Id] INT NOT NULL,
[HireDate] DATETIME2 (7) NOT NULL,
CONSTRAINT [PK_Teachers] PRIMARY KEY CLUSTERED ([Id] ASC),
CONSTRAINT [FK_Teachers_People_Id] FOREIGN KEY ([Id]) REFERENCES [dbo].[People] ([Id])
);

Tables that map to derived types also store a foreign key that joins the derived table with the base table.