On Mon, Dec 1, 2008 at 5:42 AM, Charles F. Munat <[EMAIL PROTECTED]> wrote:
> > Doing nested sets in your Lift snippet is pretty easy with JPA. I'm > doing it on one of my Lift sites. One thing to remember, however, is to > use a transaction. You don't want to do half the update to the tree and > then have another use start an update before you've finished yours (or > have half of it fail). Once it gets messed up it's a disaster. > > Nested tables are good when you have lots of lookups, particularly > pulling ancestors or descendants, but not too many updates. Inserts are > not too bad, but if you have to move whole subtrees, that's a pain. > > I'm using one for, among other things, a forum, so messages once posted > never get moved. Nested sets work great for that. > > But, honestly, it is astonishing to me that Java, Scala, and especially > JPA/Hibernate apparently have no built in features for creating, > manipulating, and persisting trees (and networks are even less > supported). These problems were largely solved long ago. Why aren't they > built in? Charles, I hate to say this, but Hibernate is Open Source, so you can add the support for it. :) > > > Rails has a cool set of plugins -- acts_as_list, acts_as_tree, and > acts_as_nested_set -- that make it drop-dead simple to implement trees. > I was thinking that it would be cool if Lift had something similar but > maybe as a trait? So my object extends ActsAsNestedSet? > > I tried to do this in Hibernate, but it would require the object to > query for other similar objects. For example, if I had a Node object > that I was persisting, I'd like to do > > class Node extends ActsAsNestedSet > > And then define methods such as moveToChildOf(node), moveToFirst, > moveBack, moveForth, moveToLast, etc. But it appears that you need an > EntityManager to run a query, and there doesn't seem to be a way to go > backwards from the Node to the EntityManager (probably because, if I > understand it correctly, there will only be one copy of a given Node in > memory but there may be multiple EntityManagers?). > > So I put the code in the snippet. > > If you come up with any clever solutions, I'd love to hear them. > > Chas. > > Tim Perrett wrote: > > Hey Derek, > > > > I've been playing around with this nested set stuff today - it's pretty > > cool. I nearly have an app working that uses the db for page content and > > maps the urls out however you want, it's pretty neat. > > > > I have a slight HQL problem with subquery's but I'll post that 2mro :-) > > > > Thanks for all your help > > > > Tim > > > > Sent from my iPhone > > > > On 30 Nov 2008, at 23:37, "Derek Chen-Becker" <[EMAIL PROTECTED] > > <mailto:[EMAIL PROTECTED]>> wrote: > > > >> IIRC, JPA has support for batch updates, but I'm not 100% positive on > >> that. I'll have more time tomorrow to look at this if you'd still like > >> to discuss it. > >> > >> Derek > >> > >> On Sun, Nov 30, 2008 at 12:32 PM, Tim Perrett < > >> <mailto:[EMAIL PROTECTED]>[EMAIL PROTECTED] > >> <mailto:[EMAIL PROTECTED]>> wrote: > >> > >> > >> Derek, > >> > >> Those links are *extremely* good - thank you! I've not really done > >> anything with tree structures before and they are really, really > >> usefull. > >> > >> One thing however, it appears the optimal way of doing the insert > >> queries would be with stored procedures, however that im not down > >> with, and ideally, i want to keep database vendor independent (part > of > >> the point of using JPA) so ideally i want to suck that up into the > >> application logic. For instance: > >> > >> --START > >> SELECT @myRight := rgt FROM nested_category WHERE name = > >> 'TELEVISIONS'; > >> > >> UPDATE nested_category SET rgt = rgt + 2 WHERE rgt > @myRight; > >> UPDATE nested_category SET lft = lft + 2 WHERE lft > @myRight; > >> > >> INSERT INTO nested_category(name, lft, rgt) VALUES('GAME CONSOLES', > >> @myRight + 1, @myRight + 2); > >> --END > >> > >> This makes use of variables, and i could of course do this via 3 > >> separate queries in JPA, but i wondered if there was a neater way to > >> construct this type of thing with JPA? > >> > >> Any advice you guys have is most welcome > >> > >> Cheers > >> > >> Tim > >> > >> > >> On Nov 30, 2:27 pm, "Derek Chen-Becker" <[EMAIL PROTECTED] > >> <mailto:[EMAIL PROTECTED]>> wrote: > >> > If you're heavily skewed towards reads and not writes (as it > >> seems in the > >> > case of a CMS), you might want to look at Celko nested sets: > >> > > >> > > >> <http://www.intelligententerprise.com/001020/celko.jhtml> > http://www.intelligententerprise.com/001020/celko.jhtml > >> > > >> < > http://dev.mysql.com/tech-resources/articles/hierarchical-data.html> > http://dev.mysql.com/tech-resources/articles/hierarchical-data.html > >> > > >> > A little more work on updates than an adjacency model, but a lot > >> more > >> > efficient. > >> > > >> > Derek > >> > >> > >> > >> > >> > > > > > > > > > -- Viktor Klang Senior Systems Analyst --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "Lift" group. 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