Question

Cant copy an object even before db.SubmitChanges()

Asked by: Bigi_Slick

Hi,  I got an unusual problem
My mission is to copy the User object before it been updated on the db
and see the changes (before and after).
My code goes like this:

User user = db.Users.First(p => p.ID == id);         // gets the user object from the db
UpdateModel(user, collection.ToValueProvider());   // update the values from the FormCollection

User user_Before_Been_Updated = db.Users.First(p => p.ID == id);  //this shows the same values as user does. even though that I didnt submit the changes to the db

db.SubmitChanges();  //This should update it to the db

what should I do?



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Asked On
2009-11-04 at 01:15:02ID24870168
Tags

Linq

,

MVC

,

C#

,

ASP.NET

Topics

C# Programming Language

,

Language Integrated Query - LINQ

,

Programming for ASP.NET

Participating Experts
2
Points
500
Comments
7

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Answers

 

by: sedgwickPosted on 2009-11-04 at 01:34:26ID: 25737657

what's the implementation of db.Users?
is it property which get users table from DB?
what's the implementation of UpdateModel()?

 

by: Bigi_SlickPosted on 2009-11-04 at 01:57:39ID: 25737768

Db is a DataContext file (Linq to SQL class).
db.Users is the Users table
User user = db.Users.First(p => p.ID == id); // gets the user from the db and put it into a User class

UpdateModel() is a MVC function which updates a specific class from a collection of names and values
http://msdn.microsoft.com/en-us/library/dd470185.aspx

 

by: LabelsoftPosted on 2009-11-04 at 05:22:43ID: 25738994

Bigi slick,

To me it seems this problem is a very basic error you might fall in to when you have thought too long and hard about the problem. ;)

You did not instantiate in the above code a new instance of your User. What you did there is pass a reference to the exact same instance of the user. You know? The whole stack/heap thing?

You need to instantiate a brand new instance of User object and then populate the properties of this object with values of the instance you have a reference to with this piece of code: db.Users.First(p => p.ID == id).

I hope that helps.

 

by: Bigi_SlickPosted on 2009-11-05 at 01:24:04ID: 25747818

Hi labelsoft,
Actually, I tried to do it, I dont know why I didnt mention it before.
This is what I did:

User user = db.Users.First(p => p.ID == id);
User newOne = new User();
newOne = db.Users.First(p => p.ID == id);
UpdateModel(user, collection.ToValueProvider());

//still newOne and user have the same values
I really dont understand why it happens
I need somehow to clone the user object before I call UpdateModel

 

by: LabelsoftPosted on 2009-11-05 at 01:39:54ID: 25747884

Yeah, because you still didn't do what I said. :)

I don't know which properties your user has so I guess something like an ID and a Name? Rewrite the code like this:

User user = db.Users.First(p => p.ID == id);
User newOne = new User();
newOne.ID = user.ID;
newOne.Name = user.Name;
UpdateModel(user, collection.ToValueProvider());

Or (gotta love initalizers) we can nowadays write it like this:

User user = db.Users.First(p => p.ID == id);
User newOne = new User() {.ID = user.ID, .Name = user.Name};
UpdateModel(user, collection.ToValueProvider());

If any of my syntax is wrong it's because I am a VB guy by the way.

 

by: Bigi_SlickPosted on 2009-11-05 at 04:34:31ID: 25748742

hi labelsoft,
I was hoping for something more dynamic.
after all, there are lots of properties that might be added later on and I want to make the same thing also to othe objects.

I'll accept your comment as solution.
if you have any other dynamic idea, please share

 

by: LabelsoftPosted on 2009-11-05 at 05:44:59ID: 25749323

Well your cloning idea wasn't that far fetched actually.

You can implement the interface IClonable in your user object and then implement the Clone method en then put in the exact same code we discussed earlier. So:

Dim u as new User {.ID = Me.Id, .Name = Me.Name}

You can check out the details here I suppose:
http://msdn.microsoft.com/en-us/library/system.icloneable.clone.aspx

That is the most dynamic "easy" solution that I can come up with out of the top of my head.

Hope that helps.

20120131-EE-VQP-002

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