CODEWITHSUNDEEP
c#entity-framework
user3530012
user3530012

Reputation: 740

Change object type in EntityFramework

I'm working on a desktop application using C# and EF6. For some reasons (One would be the complexity of the structure of the models) I've decided to use only on DbContext for the whole project, instead of create and dispose every time I need to add, update, delete or fetch any data.

Let's say I have 2 Models

public class Student
{
    public int Id { get; set; }
    public string Name { get; set; }
}

public class CollegeStudent : Student
{
    public string Course { get; set; }
}

I have an ObservableCollection in the ViewModel and instantiate it after I add an item to the collection.

I add and Student object to the database in the following way

public void AddStudent()
{
    var obj = new Student() { Name = "Mike" };
    _context.Set<Student>().Add(obj);
    StudentCollection = new ObservableCollection<Student>(_context.Set<Student>().ToList());
}

And when I want to change the type of the Student to the CollegeStudent I use the following piece of code

public void AddCollegeStudent(CollegeStudent obj)
{
    var original = _context.Set<Student>().Find(obj.Id);

    var obj = new Student() 
    {
        Id = original.Id,
        Name = original.Name,
        Course = "Some Course",
    }
    _context.Database.ExecuteSqlCommand("INSERT INTO CollegeStudent (Id, Course) VALUES (obj.Id, '" + obj.Course + "');");

    StudentCollection = new ObservableCollection<Student>(_context.Set<Student>().ToList());
}

It perfectly works and insert the CollegeStudent details in the database but when getting the list of students from the database it throws the following exception:

All objects in the EntitySet 'Students' must have unique primary keys. However, an instance of type 'CollegeStudent' and an instance of type 'Student' both have the same primary key value, 'EntitySet=Students;Id=4'

Upvotes: 0

Views: 463

Answers (2)

Devon Burriss
Devon Burriss

Reputation: 2532

I realise you are likely trying to keep things as straightforward as possible but it might be worthwhile separating your concerns sooner rather than later.

Assuming this is a XAML UI you could make use of a framework like MVVM Light or Prism

If not the basics are you want some kind of mediator (http://www.blackwasp.co.uk/mediator.aspx)

So the idea is you will have some kind of service class that makes a call to save the data, then raises the message/event saying that the data was updated. You would register a handler for when that event is raises to update the view model accordingly.

Hope this makes sense.

Upvotes: 0

D Stanley
D Stanley

Reputation: 152626

I've decided to use only on DbContext for the whole project, instead of create and dispose every time I need to add, update, delete or fetch any data.

There's your problem...

This is one reason why you shouldn't you a single DbContext for an entire app - changes to underlying data can make the data in your context invalid. Contexts are meant to be created and disposed with every DB operation. They are lightweight so creating lots of them shouldn't be a big problem.

Upvotes: 2

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