使用基于抽象类的子EF实体属性

Question

I am using Entity Framework 6 Code First, with c#4.0 and Visual Studio 2012. The following are code snippets.

I have a base abstract class:

public abstract class Person
{
    public int PersonID { get; set; }
    public String Name { get; set; }        
}

from which I derive three entities:

public class Contact : Person
{
    public Nullable<int> NHSTrustID { get; set; }
    public virtual NHSTrust NHSTrust { get; set; }        
}

 public class User : Person
{
    public int NHSTrustID { get; set; }
    public virtual NHSTrust NHSTrust { get; set; }
}

public class Notifier : Person
{
    public int NotifierTypeID { get; set; }
    public virtual NotifierType NotifierType { get; set; }        
}

The entities are declared as:

public DbSet<Person> Persons { get; set; }
public DbSet<User> Users { get; set; }
public DbSet<Contact> Contacts { get; set; }
public DbSet<Notifier> Notifiers { get; set; }

so that the Persons entity can be queries as well.

A person who could belong to any of the three derived entites. I need to know if a person has the property NHSTrust . ( Users must have an NHSTrust , for Contacts it is optional and Notifiers don't have an NHS Trust.

I can see how to do it with lots of code, but is there an elegant way of doing this?

UPDATE My current 'solution' is:

var tempPerson = dbContext.Persons.Find(personID);  
NHSTrust nHSTrust = null;

if (tempPerson is Contact)
{
    nHSTrust = dbContext.Contacts.Find(personID).NHSTrust;
}
else if (tempPerson is User)
{
    nHSTrust = dbContext.Users.Find(personID).NHSTrust;
}
if (nHSTrust != null) { // do something}

Is there a way of doing this with just a single trip to that database.

Answer 1

Create another level of your inheritance hierarchy:

public abstract class TrustablePerson : Person
{
    public NHSTrust NHSTrust { get; set; }
}
public class Contact : TrustablePerson
{
    public Nullable<int> NHSTrustID { get; set; }
}
public class User : TrustablePerson
{
    public int NHSTrustID { get; set; }
}

Now you can add a TrustablePerson to your context that gets items of that type.

Answer 2

This seems like an abstraction-level issue, where an abstract class is probably unneeded. It doesn't really save you anything in such small classes. Additionally, because some of the derived types are so similar, if you really want to abstract this, you should probably use an interface.

However, to answer your question, you can use the is operator, or even use linq. It will be fairly long and a bit messy. Really, though, your problem is the abstraction.