R – NHibernate: can I prevent setting of auditing fields from making an object dirty

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A common thing that many people do with NHibernate is setting of auditing fields (e.g. CreatedDate, LastModifiedDate) when they save an entity. There are lots of blog posts on how to do this sort of thing using NHibernate events.

This one uses the SaveUpdate event:
http://www.codinginstinct.com/2008/04/nhibernate-20-events-and-listeners.html

This one uses the PreInsert and PreUpdate event:
http://ayende.com/Blog/archive/2009/04/29/nhibernate-ipreupdateeventlistener-amp-ipreinserteventlistener.aspx

SaveUpdate feels like a better place to do this, and I'm guessing that this is where most people tend to do it. Here's the problem: I have a one-to-many relationship set up as cascade="save-update". When I save the parent entity, it runs all of the child entities through the SaveUpdate event. But when I set the audit fields there, it just made the child entity dirty! Now it always updates every row in the child collection.

I can move the auditing to PreInsert/PreUpdate, but then I have other problems (I'm using NHibernate Validator for validation, which sets constraints on columns for the schema export, so NHibernate doesn't even get to PreInsert because the audit fields are null and the db columns are not-null columns).

To me the simplest fix would be if there were a way to set the audit fields in SaveUpdate without it making my object dirty. I don't know if that's possible though.

Any suggestions? I'm sure someone has run into this problem.

Best Answer

I would say, SaveUpdate is not the right place, since it runs into this trigger even if it does not save or update the entity.

You need a trigger which is only called when the entity is already declared dirty by NH. This is most probably PreInsert and PreUpdate.

I see the problem you get with the validator. the problem is not that NH thinks that changes on these properties make the entity dirty. The problem is that the validator validates this properties too early.

If you can't parameterize the validator to skip these values, you could probably solve it with this workaround:

On SaveUpdate: check if the values are null, set it in this case to make the Validator happy. They are only null when the entity is a new instance, then it will be stored anyway.
On PreUpdate and PreInsert: always set it, because the entity had changed.

Related Solutions

R – NHibernate set properties automatically on save

Interceptors can do this:

http://www.nhforge.org/doc/nh/en/index.html#manipulatingdata-interceptors (9.10, link doesn't always work).

Cut/paste from the document:

public class AuditInterceptor : IInterceptor
{
    ...

    public boolean OnSave(object entity,
                          object id,
                          object[] state,
                          string[] propertyNames,
                          IType[] types)
    {
        if ( entity is IAuditable )
        {
            for ( int i=0; i<propertyNames.Length; i++ )
            {
                if ( "CreateTimestamp" == propertyNames[i] )
                {
                    state[i] = DateTime.Now;
                    return true;
                }
            }
        }
        return false;
    }

}

OnFlushDirty() could be used for UpdatedDate.

You are going to want every entity that has a CreatedDate/UpdatedDate to implement an interface so that the interceptor can check whether it should act (in the provided example it is IAuditable).

Edit

Just noticed chapter 11: Interceptors and events. You can inherit from EmptyInterceptor which makes things easier if you only need to override some of the methods.

I've never used Events.

Inserting parent and children with NHibernate at the same time

NHibernate is not magic. It is only an ORM, if your children do not have their reference set to the parent, why would it assume that because the parent has a list of children that the childrens' reference, in turn, SHOULD be a reference to the parent?

I believe you answered your own question when you stated that the Child entities don't have their Parent property populated (which means it is null, which means NHibernate would be attempting to save a 'null' value for the Parent id in your Child table).

If you were to use these objects WITHOUT NHibernate it would make sense that the Parent reference needs to be set on the Child entities when they are added.

EDIT: This is for the case where you have specified 'Inverse' on your mapping. If you were to remove this call to 'Inverse,' it should work the way you wanted it to work as Inverse states that the other end (the child entity) is responsible for keeping track of the relationship. This means you need to set the reference of the parent on the child manually.

However, removing the Inverse statement would cause the Child(ren) to be saved, the parent to be saved, then the Child(ren)'s parent id to be UPDATED. Since you have a null constraint on the parent id, this means it still won't work as it would initially insert the Child with a parent id of null.

The two solutions would be to either remove this constraint, or to just add a method to the Parent called AddChild:

public void AddChild(Child childObj)
{
    childObj.Parent = this;
    Children.Add(childObj);
}

Add another method called RemoveChild:

public void RemoveChild(Child childObj)
{
    if (Children.Contains(childObj))
    {
        Child.Parent = null;
        Children.Remove(childObj);
    }
}

Then, just use those methods for adding/removing children.

Best Answer

Related Solutions

R – NHibernate set properties automatically on save

Inserting parent and children with NHibernate at the same time

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