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From Jim Klo <jim....@sri.com>
Subject Re: Tree like structures in CouchDB
Date Fri, 22 Feb 2013 22:42:37 GMT


> I wanted to give my feedback about what I've learned in this area.
> 
> First, I don't use the doc _id at all for sorting docs. It solves one single use-case,
but fails if you have others, so instead, I do this:
> 
> Every doc, whether the parent or child has identifying information. So a child might
contain the parent id, thread id, etc. Parent doesn't need to know about it's children so
it doesn't matter, as those can be pulled in a single view query.
> 
> Say I want to do something as originally stated, I'd create a view where I emit([parent_id,
next_level_id, next_level_id], null) with default values for the latter nested levels being
0 by default. When I query the view, I get back a result set that would look like the following.
> [
> {"id":"0f1e244b14452a884f3dfa5b4086f793","key":[1, 0, 0],"value":null}, <- parent
> {"id":"27f4c6bb9bcaad331e68f80629bffa6e","key":[1, 1, 0],"value":null}, <- first level
> {"id":"46c17a23254c2dcce0860b4c398e0009","key":[1, 1, 1],"value":null}, <- first item
in first level
> {"id":"95903e4c2e2cbb5e2dfbc934adf6095f","key":[1, 1, 2],"value":null} <- second item
in first level
> ]
> 

you would still need to track ancestry in most cases,… the second solution makes that possible…
also your example only works for a single 'giant' tree, unless I'm missing something… and
not a forest. I'm also not seeing  how you would get all the nodes without having to execute
a query for every node on the tree - which is pretty inefficient IMHO

also as others have noted - keeping track of an independent serial, for the sake of just ordering
the tree, with concurrency would be a real challenge; which is why I use serial ID's.


> To pull the entire thread based on the parent query is simply startkey=[1,0,0]&endkey=[1,{}]

then is your parent_id, really a root_id?  Then I'm really confused how you would use this
with trees at all…  I'm not sure how you model as I'd get duplicates from which I could
never use to reconstruct the tree:

- A					A root				[A, 0, 0]	
	- B				1st child of A			[A, 1, 1]	
		- C			1st child of B			[A, 2, 1] ???
		- D			2nd child of B		[A, 2, 2] ???
	- E				2nd child of A		[A, 1, 2]
		- F			1st child of E			[A, 2, 1] ??? 
			-G		1st child of F			[A, 3, 1]
		- H			2nd child of E		[A, 2, 2] ??? 
	

> 
> The advantage of this approach is simply that say I want to display a list of all posts
by user for a specific thread, I can create a view where I emit([parent_id, user_id, comment_id],
null)

you could do this with either approach, it's not really an advantage.

> 
> This gives the ability to pull a specific comment for a user based on user_id and thread_id,
or an entire list of comments based on user_id. These sorts of indexes are very cheap and
flexible. You never have to mess with creating your own custom id system. Of course, the tradeoff
is that you have to do your own conflict resolution for async operations with thread ids if
you want them to increment. Better solution here is to use both timestamp and user_id for
the actual comment to ensure it is unique and still sorts well.
> 

again serial id's solve that (not the default UUID's couchdb issues, AFAIK they are not incremental,
however I could be wrong), is there a reason you want to avoid having a smart id?

FWIW: this all seems like deja vu http://markmail.org/search/list:org.apache.couchdb.user+modelling+a+tree+in+couchdb+date:201112-201201+



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