Add support for many types per table relationships

[simon-reynolds]

These models were related types are all stored in one database table using a discrimator column would traditionally be modelled in C# using inheritance

They should be modelled as Discrimated Unions in F#

Example:

C#

public abstract class Person {}

public class Employee : Person {}

public class Manager : Person {}

F#

type Person =
| Employee of Employee
| Manager of Manager