Entity Framework Validation & usage

Question

I'm aware there is an AssociationChanged event, however, this event fires after the association is made. There is no AssociationChanging event. So, if I want to throw an exception for some validation reason, how do I do this and get back to my original value?

Also, I would like to default values for my entity based on information from other entities but do this only when I know the entitiy is instanced for insertion into the database. How do I tell the difference between that and the object getting instanced because it is about to be populated based on existing data? Am I supposed to know? Is that considiered business logic that should be outside of my entity business logic?

If that's the case, then should I be designing controller classes to wrap all these entities? My concern is that if I deliver back an entity, I want the client to get access to the properties, but I want to retain tight control over validations on how they are set, defaulted, etc. Every example I've seen references context, which is outside of my enity partial class validation, right?

BTW, I looked at the EFPocoAdapter and for the life of me cannot determine how to populate lists of from within my POCO class... anyone know how I get to the context from a EFPoco Class?

Answer 1

Concerning your first question, I would simply implement the changes to the associations as business logic. For example, if you add a Teacher class with multiple Student, do not add students like

aTeacher.Students.Add(new Student)

instead, create a AddStudent method

public Student AddNewStudent(string name, string studentID)
{

    Student s = new Student( name, studentID);
    s.Teacher = this; // changes the association
    return s;
}

That way you have full control on when associations are changed. Of course that what prevents another programmer from adding a student directly? On the Student side, you can set the Teacher setter to private (and change the constructor to accept a teacher or similar). On the teacher side, how to make the Students collection non-insertable? I'm not certain... maybe transforming it in a custom collection that doesn't accept inserts.

Concerning the second part of your question, you could probably use the OnVarNameChanging events. If the EntityState is 'New' then you can apply your logic that fetches the real values.

There is also an event that fires when you save changes (OnSavingChanges?) that you could use to determine which objects are new and set some values.

But maybe the simplest solution is to always set the defaults in the constructor and they will get overwritten if the data is loaded from the DB.

Good luck

Answer 2

Create a factory that produces instances for you depending on your need like:

getStudent(String studentName, long studentId, Teacher teacher) {
    return new Student(studentName, studentId);
}

getStudentForDBInseration(String studentName, long studentId, Teacher teacher) {
    Student student = getStudent(studentName, studentId);
    student = teacher;
    //some entity frameworks need the student to be in the teachers student list
    //so you might need to add the student to the teachers student list
    teacher.addStudent(student);
}

Answer 3

It's a serious lack not having an AssociationChanging (that inherits from CancelEventArgs) event.

It bothers me also very much, therefore I reported this to Microsoft Connect Please vote here!

And BTW, I also think this is also stupid that the PropertyChangingEventArgs doesn't inherit CancelEventArgs, since cancelling with an exception is not always the elegant solution, besides, throwing exceptions cost more performance than calling the OnPropertyChangingEvent then check for the returned e.Cancel, so does it cost less than raising the PropertyChangingEvent, which you anyway call them both.
Also an exception can be thrown at the handler anyway instead of marking e.Cancel as true, for those who insist to go the Exception way. Vote Here.