Workflow for data from Backbone through NHibernate

Question

This question isn't code-centric, it's a question about idioms. I'm using Backbone/Marionette on the front and C#, NHibernate in the back.

I've got a few tables mapped and working for creates and updates. For simplicity, say I've got two tables, a parent and child, and the child has potentially many rows. The child reference to the parent is not-nullable, so I've got an Inverse relationship, and that's all working. The flow of data from Backbone to the Controller to NHibernate is pretty simple, and I'm using ISession.SaveOrUpdate() at the end. I can post mappings and so forth if necessary. I will say that the mappings Fluent NHibernate generates uses a Bag on the parent.

Here's a concrete example of the situation I'm trying to understand. Say I've got a parent entry with two rows in the child table. I manipulate the data so that one of the children is removed, but no other changes are made. Javascript sends over an object "tree" with the parent entry and the one child row that's left. The mappings are all handled fine, but the sql that's generated is a bunch of (unnecessary, but whatever) update statements. What I would like to happen instead is that NHibernate notices that there is only one child relationship in this new object, but there are two children in the actual database, and then NHibernate deletes that other child. The 'cascade-delete-orphans' option is not working, because the other child isn't actually being orphaned. It still has a reference to the parent in the fk column, and that column is non-nullable anyway, which is why I used the Inverse mapping option.

Is that possible to setup in the mappings? If not, what is a good way to tackle this situation?

Answer 1

Since you are sending an object from the client side, and then create the entity from that object and try to persist, NHibernate will not automatically delete the child entity since it does not know the child object is deleted (it only see you are only try to update one parent entity and a child entity), which is correct in my opinion. For example if you want to just update the parent entity field, then you have to load entire object graph to do it, otherwise NHibernate will delete all children since they are not loaded.

What you should do here is to load the parent entity, and remove missing child entity(s) deleted from it and then persist (instead of mapping the entity), code should look like following,

void Update(ParentDto parentDto){
    Parent parent = _session.Get<Parent>(parentDto.Id);
    //update parent fields

    var childRemoved = //find removed child from parent;
    parent.Children.Remove(childRemoved);

    _session.SaveOrUpdate(parent);
}