http://ayende.comAyende @ RahienCopyright (C) Ayende Rahien 2004 - 2021 (c) 202660Sternr, Nope, the problem with doing those things in the mapping is that they are global. See my post about the different scenarios and how they do not apply http://ayende.com/4573/nhibernate-is-lazy-just-live-with-it#comment23http://ayende.com/4573/nhibernate-is-lazy-just-live-with-it#comment23Sun, 08 Aug 2010 20:05:32 GMTMaybe I'm missing something, but you seem a bit harsh on lazy=false. In cases where you know related entities are needed, lazy-loading has massive performance implications, especially in cases of smart client applications where server connectivity comes at great cost. Your client was right in mapping it's entities with lazy="false", his only mistake was not setting the fetch mode. I agree the decision not to lazy load has great implications, and should be dealt with care, but there is a great distance from getting the goosebumps over it. The main lesson we ought to learn from your client is not " we *heart* lazy", but "NHibernate is not magic". or in short RTFM ;) http://ayende.com/4573/nhibernate-is-lazy-just-live-with-it#comment22http://ayende.com/4573/nhibernate-is-lazy-just-live-with-it#comment22Sun, 08 Aug 2010 15:35:17 GMTI agree @sql. For straight entities stored in the database, using nhib is great. Insert, update and query are very easy. As soon as you have any other query (most others), writing a sql statement and mapping that using linq is far easier. http://ayende.com/4573/nhibernate-is-lazy-just-live-with-it#comment21http://ayende.com/4573/nhibernate-is-lazy-just-live-with-it#comment21Sat, 07 Aug 2010 08:13:51 GMTDmitry: Yes, null. Because you didn't fetch it and lazy loading isn't loading it. That's why a proper eager loading strategy is essential, e.g. through prefetch paths, Includes or what have you. (so you'd issue 3 queries, one for each entity type and merge them internally) http://ayende.com/4573/nhibernate-is-lazy-just-live-with-it#comment20http://ayende.com/4573/nhibernate-is-lazy-just-live-with-it#comment20Fri, 06 Aug 2010 16:54:47 GMTMark, Two reasons, backward compatibility and that there _are_ scenarios where you want that. http://ayende.com/4573/nhibernate-is-lazy-just-live-with-it#comment19http://ayende.com/4573/nhibernate-is-lazy-just-live-with-it#comment19Fri, 06 Aug 2010 07:24:31 GMTSo, playing devils advocate, if default_lazy=false is so bad, why does NH allow you to do it ? I understand in specific instances there might be a reason to override a relationship so that it isn't lazy loaded, but from whats said here there is no reason to have a default of default_lazy=false. I'm reading alot of NH stuff recently which says "x is bad, don't do it", seems the tweaking and configuration of NH adds a fairly significant overhead to any project. http://ayende.com/4573/nhibernate-is-lazy-just-live-with-it#comment18http://ayende.com/4573/nhibernate-is-lazy-just-live-with-it#comment18Fri, 06 Aug 2010 07:11:03 GMTJohn, The way NHibernate work, it has to process each loaded entity immediately, because that entity is being exposed to user code. So before the entire list is done, it has to process all the work for the entity. It doesn't have the chance to look at everything. The problem then becomes that you set lazy=false, didn't set fetch (so it is at the default select), which instruct NH to issue a separate select. http://ayende.com/4573/nhibernate-is-lazy-just-live-with-it#comment17http://ayende.com/4573/nhibernate-is-lazy-just-live-with-it#comment17Wed, 04 Aug 2010 19:26:15 GMTFrans, If you set the mapping to "lazy=true" and do not eagerly load parts, what would be the Parts property set to? Setting it to null is something EF 1.0 tried to do and is a bad idea because you end up with a broken model. Do you throw an exception? I am not sure I like it either. http://ayende.com/4573/nhibernate-is-lazy-just-live-with-it#comment16http://ayende.com/4573/nhibernate-is-lazy-just-live-with-it#comment16Wed, 04 Aug 2010 19:10:58 GMTI'm surprised NHIbernate doesn't efficiently eagerly load graphs via joins. I couldn't live without this feature in EF. The answer to Damien seems a bit weird. Why wouldn't NHibernate not know about further polymorphic collections or other nonsense? Aren't these things set up in the mapping files? http://ayende.com/4573/nhibernate-is-lazy-just-live-with-it#comment15http://ayende.com/4573/nhibernate-is-lazy-just-live-with-it#comment15Wed, 04 Aug 2010 17:58:33 GMTSo what is the answer? What should he actually have done in this situation to load all that data? It is a problem we come across a lot. http://ayende.com/4573/nhibernate-is-lazy-just-live-with-it#comment14http://ayende.com/4573/nhibernate-is-lazy-just-live-with-it#comment14Wed, 04 Aug 2010 16:47:03 GMTFrans, Also note that NH make a distinction between lazy=true/false and how to eagerly load things fetch=join/select http://ayende.com/4573/nhibernate-is-lazy-just-live-with-it#comment13http://ayende.com/4573/nhibernate-is-lazy-just-live-with-it#comment13Wed, 04 Aug 2010 16:04:30 GMTFrans, It has to load that because the mapping explicitly told it to. http://ayende.com/4573/nhibernate-is-lazy-just-live-with-it#comment12http://ayende.com/4573/nhibernate-is-lazy-just-live-with-it#comment12Wed, 04 Aug 2010 16:04:07 GMTMakes writing standard SQL seem appealing. http://ayende.com/4573/nhibernate-is-lazy-just-live-with-it#comment11http://ayende.com/4573/nhibernate-is-lazy-just-live-with-it#comment11Wed, 04 Aug 2010 16:04:05 GMTDamien, Because it is too early for NH to know. Imagine a polymorphic query that load some things with different collections, for example http://ayende.com/4573/nhibernate-is-lazy-just-live-with-it#comment10http://ayende.com/4573/nhibernate-is-lazy-just-live-with-it#comment10Wed, 04 Aug 2010 16:03:17 GMTFrank, I didn't write the blog, and the blog isn't using NH, so I have no idea http://ayende.com/4573/nhibernate-is-lazy-just-live-with-it#comment9http://ayende.com/4573/nhibernate-is-lazy-just-live-with-it#comment9Wed, 04 Aug 2010 16:02:07 GMT João P. Bragança, Read the post, you shouldn't use lazy=false If you are using reporting only model with no associations, then lazy=false is still isn't right. It wouldn't do anything, anyway http://ayende.com/4573/nhibernate-is-lazy-just-live-with-it#comment8http://ayende.com/4573/nhibernate-is-lazy-just-live-with-it#comment8Wed, 04 Aug 2010 16:01:41 GMTJason, It is eventually perf. It is also limiting what NH can do. http://ayende.com/4573/nhibernate-is-lazy-just-live-with-it#comment7http://ayende.com/4573/nhibernate-is-lazy-just-live-with-it#comment7Wed, 04 Aug 2010 16:00:47 GMTI don't really see why NHibernate needs to fetch Parts etc. when the query doesn't refer to it. In fact, by doing so (and thus resulting in 400 queries) it is doing lazy loading (load on demand) while there's actually no need for it. If you switch off lazy loading, like in the mapping file, the developer should instead EAGER load the data of the graphs. The queries generated now are the same as when he'd traverse the rows and access the Parts collection, wouldn't it? He also could have solved this by using a fetch strategy btw, not ideal either (always eager loading isn't ideal). http://ayende.com/4573/nhibernate-is-lazy-just-live-with-it#comment6http://ayende.com/4573/nhibernate-is-lazy-just-live-with-it#comment6Wed, 04 Aug 2010 15:36:22 GMTWhy isn't NHibernate's second query set a single query with an IN clause? Yes, it would still return too much data but at least there would be less overhead/latency. [)amien http://ayende.com/4573/nhibernate-is-lazy-just-live-with-it#comment5http://ayende.com/4573/nhibernate-is-lazy-just-live-with-it#comment5Wed, 04 Aug 2010 14:46:47 GMTWhen you load this page, do you have a content -> comment association and do you load the comments lazily? or do you load the content and the comments at once and display it? http://ayende.com/4573/nhibernate-is-lazy-just-live-with-it#comment4http://ayende.com/4573/nhibernate-is-lazy-just-live-with-it#comment4Wed, 04 Aug 2010 14:33:47 GMTCould you talk more about which scenarios where lazy=false should be used? For example I am using nhibernate for a reporting only model, no associations. Does lazy=false make sense here? http://ayende.com/4573/nhibernate-is-lazy-just-live-with-it#comment3http://ayende.com/4573/nhibernate-is-lazy-just-live-with-it#comment3Wed, 04 Aug 2010 14:28:47 GMTIs the downside of lazy=false strictly performance in this case? (I'm not implying that performance is unimportant.) http://ayende.com/4573/nhibernate-is-lazy-just-live-with-it#comment2http://ayende.com/4573/nhibernate-is-lazy-just-live-with-it#comment2Wed, 04 Aug 2010 14:23:04 GMTI think it is a common misconception that NHibernate is simpler without lazy loading. But actually I think it is far more difficult to get right - especially when you throw in further non-lazy 'simplifications' like treating sessions as transient in your architecture and using disconnected objects (preventing you from ever enabling lazy loading again). Glad you wrote about it, now I can clobber people with a link next time http://ayende.com/4573/nhibernate-is-lazy-just-live-with-it#comment1http://ayende.com/4573/nhibernate-is-lazy-just-live-with-it#comment1Wed, 04 Aug 2010 12:33:49 GMT