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From "Erick Erickson" <erickerick...@gmail.com>
Subject Re: Querying fragments of a tree structure
Date Wed, 21 Mar 2007 13:12:49 GMT
Is it a fair restatement of your problem that you want to generate
a list of all children of a node? That's what I'm reading.....

Would it work for you to store the complete ancestry in each node?
By that I mean (from your example),

NOTE: it's no problem in Lucene to store different values for the
same field in the same document. I.e
Document doc.
doc.add("field" "value1");
doc.add("field" "value2");
writer.add(doc);...

This is equivalent (if using WhitespaceAnalyzer in this example)  to:
Document doc;
doc.add("field", "value1 value2");
writer.add(doc);

(There is a subtle difference between the two having to do
with PositionIncrementGap, but that's probably irrelevant for you
in this problem).

So what about just doing that for each parent node in your tree? So
your "ancestry" field for documents D and E have stored "C" and "A".
This is TOKENIZED, but not necessarily STORED....

Document C has only "A".

Now, finding the children of "A" reduces to something like
+ancestry:A
which you can add to your BooleanClauses if you want to also specify
other search criteria or just use by itself if you don't.


What follows is my first idea, but I think the above is a better notion.

Node A stores nothing
Nodes  B and C  store "A"
Nodex D and E store "A$C"
etc.

Now, finding all the children of A reduces to doing a WildcardTermEnum
on "A*" and, for each resulting term using TermDocs.seek(term) to find
the corresponding document.

Note a couple of things:
1> index the ancestry field UN_TOKENIZED. You don't need to store it.
1a> You could use something like this to form a Lucene Filter if you needed
to, say, find all the nodes in the tree that were children of a specified
node
AND met certain search criteria.

2> You could also just search on A*, but be aware that you may have to
deal with TooManyClauses exceptions. The TermEnum/TermDocs method
avoids that problem, but may be overkill in your situation.
2a> Lucene 2.1 allows wildcards in the first position if you do a wlidcard
search, but you need to turn that on by a call which I can't bring up from
memory.


Hope this helps
Erick

On 3/21/07, Emanuel Schleussinger <emanuel@ngine.de> wrote:
>
> Hi,
>
> first, thanks for this great a resource, and sorry if i am oversimplfying
> a few things, i am still rather new to Lucene.
>
> I have been thinking how to integrate my app with Lucene - it is a CMS
> type system that has documents organized in a tree-style layout. A few facts
> about the system:
> - Every node in the system is represented by a unique numeric id in  a
> field called "id"
> - There is one deinfed root node, and an arbitrary amount of descendants.
> - Each of the nodes on any level knows his descendants in a field called
> "child"
> - Each node also knows his parent node in a field called "parent"
>
> I am indexing all the fields from all the nodes in Lucene already, and
> thus, i can use Lucene to e.g. get all the descendant node IDs of a node
> simply by issuing a query like "id:2" and then extracting the
> multivalue-field "child".
>
> Now, here is what i am trying to solve now -- i would like to be able to
> fetch all the nodes that match a certain criteria, if they are contained in
> some fragment of the tree. To visualize:
>
> Root
> +-> A
> |   +-> B
> |   +-> C
> |       +->D
> |       +->E
> +->F
>     +->G
>
> i would like to issue a query that gives me all the nodes within "A", a
> flat list of results that contain B,C,D and E
>
> Now, since per my definition D is not directly correlated with A (it knows
> his parent C, but not that its also part of A -- only C knows that) , i was
> thinking of introducing a new field for every node into my Lucene index that
> holds a list of IDs thats trace back to the root element (in this case, the
> D node would have C and A in that field, in that order) - but it strikes me
> this may not be the most elegant approach...
>
> The above is only a simplified example, in reality, i have a tree about 10
> levels deep, with thousands of nodes, and i frequently need to surface nodes
> within a certain fragment of that tree.
>
> Is there any best practice that you ran into on how to map this elegantly
> into Lucene?
>
> Thanks a ton for any pointers,
> Emanuel Schleussinger
>
>
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