Question

Hi,

I want to track DateLastModified (DateTime) and LastModifiedBy (User) in all of my entities.

What is the most efficient way?

I currently override the OnValidate() method and update these two fields. However, when I'm creating an entity, there is an INSERT query that inserts all of the other fields of the entity, then a second query that uses UPDATE to update the DateLastModified and LastModifiedBy fields.

I'd like for all of this to be done in 1 query. One option is to override OnPropertyChanged and set the two field's values there, but I feel that maybe this will affect performance for entities with a large number of fields.

The other option is to use ITransactionEventWatcher.OnBeforeTransactionCommit(...). In this method, is it possible to get all of the entities that have been modified and are being committed? That way I could update their DateLastModified and LastModifiedBy fields. Would this lead to 1 query?

Accepted Answer

Hello

After investigations and fixes the most efficient way to implement such functionality is following:

public abstract class Person : DataObject
{
    public abstract string Name { get; set; }

    public abstract DateTime? DateLastModified { get; set; }

    protected override void OnPersist()
    {
        DateLastModified = DateTime.Now;
    }
}

Obvious that it is most straigtforward approach and now it works. While there still possible situations (two cases) when object creation completes with two queries DataObjects will try to use single INSERT whenever it is possible. Let me explain when DataObjects will perform two queries:

The instance being persisted contains ReferenceFields and CreateForeignKeys database option is applied: Newly created instances may refer each other so foreign key conflicts are possible if a sequence of insertions is inproper. We didn't use topological sort in this case because two stage update is simple to implement.
The instance being persisted contains an ExternalField like e.g. DataObjectCollectionField. This field is [LoadOnDemand] and LoadOnDemandThreshold is greater than zero: FastLoadData in this case must contain the ExternalField content if a size of the content is lower than threshold. So three stages are required. 1) Persisting ExternalField owner; 2) Persisting ExternalField itself; 3) Calculating and persisting FastLoadData. The first and the third stages will correspond to pair of INSERT+UPDATE queries.

That is all. As you can see our OnPersist handler itself can't lead to additional queries.

Let now consider implementation of DateLastModified with OnValidate handler you currently use. It works but it is not optimal because it brings additional UPDATE query in any case. So you can get even three stage insertion in this case. The reason of such behavior is following: Before executing validate stage DataObjects begins new internal transaction (actualy places savepoint) to be able to rollback to pre-validate state in a case of error. This action, in turn, requires to flush all changes cached in memory into database.

Well. You need the new assembly to try the suggested aproach. It will soon available at our website or I can send it by e-mail.

Answer 2

While Alex Ustinov keep silence about v3.X, I'll give an answer on how to do the same in v4.X - just to keep the people informed ;)

Actually, there is a wide set of options:

Override Entity.OnInitialize (this.PersistenceState==PersistenceState.New there indicates entity is created, but not materialized) and Entity.OnSetFieldValue` in some of your base types and update necessary properties there.
Achieve the same by subscribing to Created and EntityChanging events in session events.
There are some other alternatives - e.g. you can subscribe to EntityVersionInfoChanged event, or rely on Session.Operations events.

IMO, #2 is the nicest option:

You can check if entity supports IHasCreateModifyDates there, and do necessary changes only if this is really necessary.
So to get automatic support of such updates, you should just support this interface @ certain types.
Also, you can implement IModule injecting this capability to any Session created @ your Domain. Since modules are registered via DomainConfiguration.Types, you should just register the assembly where it's declared to get the feature working.

Btw, you should think about tracking EntitySet events as well.

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