Question

I have to create an entity framework model for a badly designed database. The database uses table per type inheritance but it does so with using a PK->FK relationship, not a PK->PK relationship. E.g.

Person
   PersonID (PK)
   Name

Employee
   EmployeeID (PK)
   PersonID (FK)
   DateStarted

HourlyEmployee
   HourlyEmployeeID (PK)
   EmployeeID (FK)
   HourlyRate

Obviously this is just badly designed, but I can’t change it. Table per type inheritance in the entity framework essentially wants EmployeeID not to exist and the PK for Employee to be PersonID. Is it possible to create a model for this database, or do I choose another tool? any recommendations?

Answer 1

You will not map this as TPT inheritance because your database is configured in the way that doesn’t allow you cheating EF.

If Employee.EmployeeID is auto-generated in the database and Employee.PersonID is unique (uniqueness must be enforced in the database) you should be able (not tested) to cheat EF by simply mapping:

public Employee : Person {
    public DateTime DateStarted { get; set; }
}

This class will tell EF that Employee inherits key from Person (PersonID) and you will hide the real key from EF – this should work if the real key is auto-generated.

The problem is your next level of inheritance which breaks this pattern. To make this work your HourlyEmployee will have to reference PersonID – not EmployeeID. EF now doesn’t know about EmployeeID existence so it even cannot map relation with HourlyEmployee.

TPT inheritance in code first has one additional limitation – PK column must have the same name in all tables.