When must we use eager loading in NHibernate? What is it's usage?
One usage is when you will cache or store an object graph (in ASP.NET Cache for instance). If you don't store the whole graph, you would be missing information on a detached object. You can reattach objects of course, but that would probably be a new roundtrip to the database anyway.
If you don't eager load your collections, you would need to touch every one of the to invoke the lazy fetch. In those cases, an eager fetch is much more useful.
Maybe this presentation by Udi can help you decide.
http://www.infoq.com/presentations/Making-Roles-Explicit-Udi-Dahan
Related
is it possible in Fluent NHibernate to enable or disable Lazy loading in execution time?
In some cases when I retrieve an object I need the Many relation to be lazy loaded and in other cases I don't want.
Thanks in advance
Short answer: no.
You should map the association as lazy (this is the default), and when required, use a fetch load query.
At the moment, I'm learning more about ORM and entity states and lifecycles.
I understand what a detached entity is and how an entity can get into detached state.
However, one thing I really don't understand:
What is the use of the detach()-method offered by an entity-manager?
I mean it's one thing for an entity to become detached e.g. after serialization/deserialization.
But why would one want an entity to become detached by explicitly calling the detach()-Method?
Does anyone have some examples of usefull scenarios?
It saves resources, so if you're going to have entities that are being used in a read only situation, you can detach them.
I have set NHibernate to not lazy load for my entities. But sometimes when I do queries I don't want all the children of the children to be loaded. The mapping is set up by Fluent NHibernate.
Is there any way when writing the sql for the query to specify which columns to lazy load?
I believe, you're using the wrong approach. Set all mappings to lazy load, and then in the queries eager load only what you really need. This way you won't kill the app.
You can override all the mappings defined in Fluent Mappings in conventions either in class mappings.
There also are different scenarios where NHibernate does the trick (for instance if you load / get one instance all the properties will be fetched as defined in mapping. If you get a list of items it will not happen unless you use Fetch method explicitly).
So could you provide some more details on your question to give an answer that is more precise?
I have entities which I would like to eagerly load , and on other ocassions lazy (or even extra lazy) load.
My mappings have no fetch mode declared in my YAML- so they use the default (lazy loading).
Currently the only way to eagerly load is to by constructing the DQL manually - and I need to update this every time I add a new entity.
Ideally I would just load the root entity and the force eager loading all the associated objects. Is there any way I can do this?
If not why (is there a reason beyond it being an unimplemented feature)?
If you want to use built-in repository methods (find(), findAll()), you're probably out of luck unless you set things to eagerly load in your annotations.
You'll probably want to use the query builder (or raw DQL) in some custom repository's method to force eager loading where you want it. Yes, you'll have to update that method as you add entities, but at least you'll always know what's going on in regards to lazy/eager loading, and you'll only need to maintain it all in one place.
I suppose the reason there's not some $eagerLoad flag to find(), etc, is because those are convenience methods for simple tasks. If you wanted to add such a flag, you'd have quickly get into situations where you'd want to limit recursive eager loading by depth. You'd also probably have to start worrying about cyclical references (any bidirectional association, for instance).
You can use setFetchMode() method of DQL to set mode.
See the documentation:
https://web.archive.org/web/20120601032806/http://readthedocs.org/docs/doctrine-orm/en/latest/reference/dql-doctrine-query-language.html
I've spent some time searching around how to configure NHibernate's FlushMode so it could only save objects that I've explicity called Save/Update/Delete, but I've figured out that I can't do that. Instead of it, I have to evict every object that I've modified (even without calling Save/Update/Delete) as I'm using NHibernate transaction management.
I understand perfectly why NHibernate have to Flush some objects before some Find operations, but I'm not worried about stale data. I see that, maybe, in some situation, flush everything that was modified and not explicity saved can be usefull, but it's not my case.
I simply want that, after commiting my session, NHibernate inserts/updates/deletes everything that I have explicitly demanded it to, and evict everything else. My question is: is this behavior just a question of "nobody stopped to implement this yet" or are there another points that would fail if this kind of behavior existed?
Thank you in advance.
Filipe
Nhibernate doesn't think that way. The distinction is between transient and persistent objects, and persistent objects are synchronized with the database when the session is flushed (an possibly at other times). Objects that are retrieved using NH are persistent and will be saved when the session is flushed without calling Save (or SaveOrUpdate) because they are already persistent. There are several options for controlling the FlushMode but nothing that would make it work the way you desire.
A potential workaround might be to retrieve objects using an IStatelessSession and handle operations through a separate ISession.
What problem are you trying to solve?
You are basically asking: "why doesn't my hammer work more like a screwdriver?"
The whole idea of the Session (among other things) is to allow automatic dirty tracking, so you don't need to worry about what was changed; only additions and non-cascaded deletions are manual.
As Jamie mentioned you can use IStateLessSession instead of ISession. It does not track anything automatically and it does not support lazy loading. You have to tell it explicitly what to insert, update and delete. It's more used for read-only and batch contexts, but it's probably what you're looking for,