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Undo delete entity

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Erik
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Erik asked on 07 Oct 2012, 09:58 AM
Is it possible to 'undo' a dbContext.Delete operation?

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IT-Als
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answered on 09 Oct 2012, 06:26 AM
Hi Erik,

I believe there are two ways of doing this, but as always it depends... on how complex the entity you deleted is.

1)
You could do a rollback, but it is rather coarse-grained, meaning.. you rollback any changes applied to the context.

2)
You could Add the entity to the Context again.

Regards

Henrik
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Doroteya
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answered on 11 Oct 2012, 02:28 PM
Hello,

Henrik, thank you for the response. I just want to explain that while you are right about the first way, while the second one is possible only if a few conditions are fulfilled.

Rollback Changes
In OpenAccess, dbContext.Delete(...) marks the object for removal and to undo it before the transaction (to roll it back) you can use dbContext.ClearChanges() (more about this method you can read here). Note that, it will cancel all pending changes from the context (including dbContext.Delete(..)) and if you intend to use it, be very careful with context management.

Adding back an object
In OpenAccess, the lifecycle of the objects you make persistent by calling dbContext.Add(...) is as follows: transient -> persistent new -> persistent new deleted or hollow. That means that when you create an object in your code, add it to the context, mark it for removal and than add it again, the second dbContext.Add(...) command will be ignored. In other words you cannot do
    dbContext.Add(someObject);
    dbContext.Delete(someObject);
    dbContext.Add(someObject);
in the same transaction.

The other scenario when you retrieve an existing object from the database, mark it for removal and then add a new object with the same property values, is possible and it looks roughly like this (using the SofiaCarRental database):
using (EntityDiagrams dbContext = new EntityDiagrams())
            {
                //Retrieve an object from the database
                string carTagNumber = "TE 9876 IK";
                Car carToDelete = dbContext.Cars.FirstOrDefault(c => c.TagNumber == carTagNumber);
 
                //Add some code to preserve the values
                //of the properties of carToDelete
                string carModel = carToDelete.Model;
 
                //carToDelete is marked for delete and the values
                //of all its properties are set to NULL
                dbContext.Delete(carToDelete);
 
                //An object with properties equal to the properties
                //of carToDelete is created
                Car carToAdd = new Car
                {
                    TagNumber = carTagNumber,
                    Model = carModel
                    //Add values for the rest of the properties of carToAdd
                };
 
                //Add carToAdd in the context
                dbContext.Add(carToAdd);
 
                //Once you save the changes the delete statement will be executed
                //and after that the new object will be added to the database with
                //the same property values.
                dbContext.SaveChanges();
            }

For additional information about the lifecycle of the objects in OpenAccess, I suggest you to check here and here.

If you have any other questions or you are trying to implement another scenario, do not hesitate to get back to us.


All the best,
Doroteya
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IT-Als
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answered on 11 Oct 2012, 02:35 PM
Hi Doroteya,

Thanks for the clarification.

That was exactly what I meant by the phrase: "it depends... on how complex the entity you deleted is" :-)

Because you are actually creating a brand new instance and just copy the properties of the deleted object.

So if your object graph is deep or have many properties per level, this is quite cumbersome - at least doing it manually (copy property by property - referened object by object)

Clone to rescue

/Henrik
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Erik
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answered on 11 Oct 2012, 03:59 PM
Thanks Henrik & Doroteya for your replies. I decided to add the items I want to delete to a separate list, and only mark them for deletion just before calling dbContext.SaveChanges.
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Doroteya
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