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+
