Entity Framework - Get identity value in SavinChanges event

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Hi all,
I was wondering if there is any way to find out what identity value an EntityObject will get in the SavingChanges event.

any help is kindly appreciated,
cheers
Andy

Posted 10-Feb-12 3:45am

hoernchenmeister

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Christian Graus 10-Feb-12 9:57am

Do you mean if a new object is added to the database ?

hoernchenmeister 10-Feb-12 10:08am

I have added a partial OnContextCreated to my context that registers the SavingChanges event.
I am doing this in order to audit the changes that have been made to the context.
Then I save the changed column values together with the DataRowState and the id for the entry.
This works fine for Modified/Deleted entries, but for Added entries I can not store the id (because it is not created at this point).
I could store 0, but that doesn't help finding the added entry later in the database.

So now I am searching for a way to get the real id of the entry that will be added.
I hope I could explain what I want to do ;)

darkDercane 10-Feb-12 11:42am

Do you mean Persist???
cos the context of the repository it have a persist method, when you call that method automatically your object is actualized with the lastest ID(only if that field is Identity in your DB) :)

hoernchenmeister 13-Feb-12 4:40am

Thanks for your comment.
I googled for a while but I couldn't find a "persist" method related to the ObjectContext. I found out about the repository pattern though.
Would you mind to point me into the right direction on where to search?
That'll be great ;)

2 solutions

Solution 2

Identity values don't exist until the row is actually inserted into the table. I've not used EF myself but if it follows the Microsoft standards then 'SavingChanges' will be fired before that happens, to allow you to validate or modify the object before it actually gets saved. That means that in this event, there won't yet be a row created in the database and therefore it is logically impossible to get the identity value (since the database engine didn't make one yet).

You need to save a placeholder, and catch the event issued when a row is successfully saved ('SavedChanges' at a guess). At that point you should be able to determine the value that was actually assigned.

Posted 10-Feb-12 5:49am

BobJanova

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hoernchenmeister 13-Feb-12 4:28am

Thanks for your help.
I guess you pointed out the problem pretty nicely. There is no option to get the id before the values are actually written to the database, which of course makes sense.
Problem is that there is no such event like "SavedChanges", only "SavingChanges" which is the place where I am currently trying to attach anything that might enable me to check the id later.
I also wasn't able to attach anything to the entity that will be saved when the SavingChanges event is fired so I can not check the id after the insert is completed (as far as I know)

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Wonde Tadesse · Accepted Answer · 2012-02-10T05:31:00

As far as you are using the latest version for EF(4.0,4.1), there is no problem getting the last inserted entity object identity value. Note The identity column value will be returned only after the inserted statement committed successfully.

1. Insert the Entity Object.
2. Save the Entity Object using database context object SaveChanges() method
3. Don't dispose the database context object.
4. Return the Entity Object from your method.

C#

/// <summary>
/// Add Type Customer entity to the repository
/// </summary>
/// <typeparam name="Customer">Type Customer entity to be added</typeparam>
/// <param name="entity">Customer  entity to be added and returned</param>
/// <returns>Type Customer  entity object</returns>
/// <remarks></remarks>
public Customer  AddRecord<Customer>(Customer entity)
{
    try {
        bool success = entityRepository.AddRecord<Customer>(entity); // Note : The repository must not dispose the database context in order to get the last identity value of the Entity Object.
        if(success)
          return entity;
    } catch (OptimisticConcurrencyException concurrencyException) {
        entityRepository.Dispose();
        throw concurrencyException;
    } catch (Exception addRecordException) {
        entityRepository.Dispose();
        throw addRecordException;
    }
    return null;
}

The AddRecord method from the repository looks something like this. Note. It just code fragment, there are lots of methods that should be contained in the repository.

C#

/// <summary>
/// Add entity to the repository
/// </summary>
/// <typeparam name="E">An Entity type object type which is going to be added</typeparam>
/// <param name="entity">The entity to be added</param>
/// <returns>True/False</returns>
/// <remarks></remarks>
public bool AddRecord<E>(E entity)
{
    this.objectContext.AddObject(GetBaseType(typeof(E)).Name.ToString(), entity);
    this.objectContext.SaveChanges();
    return true;
}