ElasticSearch - Boosting based on depth in a recursive structure - elasticsearch

Question

I am using Elastic search 2.4.4(compatible with spring boot 1.5.2).
I have a document object which has the following structure :
{
id : 1,
title : Doc title
//some more metadata
sections :[
{
"id" : 2,
"title: Sec title 1,
sections:[...]
},{
id : 3,
title: Sec title 2,
sections:[...]
}
]
}
Basically I want to make the titles in the document searchable(all document title, section titles and subsection titles at any level) and I want to be able to score the documents based on the level at which they match in the tree hierarchy.
My initial thought was using some strcture like this :
{
titles:[
{
title : doc title,
depth : 0
},
{
title : sec title 1,
depth : 1
},
{
title : sec title 2,
depth : 1
},
......
]
}
I would like to rank the documents based on the depth at which there is match(higher the depth, lower is the score).
I know the basic boosting based on the field but,
is there a way can do this in elastic search?
OR
Is it possible to do it by changing the structure?

Answer 1

Yes, you can achieve this by indexing documents in your modified format (a flat array of objects) using a Nested datatype mapping and a Function Score Query inside of a Nested Query:
PUT someindex
{
"mappings": {"sometype":{"properties": {"titles":{"type": "nested"}}}}
}
POST someindex/sometype/0
{
"titles": [
{ "title": "doc title", "depth": 0 },
{ "title": "sec title 1", "depth": 1 },
{ "title": "sec title 2", "depth": 1 }
]
}
POST someindex/sometype/1
{
"titles": [
{ "title": "sec doc title", "depth": 0 }
]
}
GET someindex/sometype/_search
{
"query": {
"nested": {
"path": "titles",
"score_mode": "max",
"query": {
"function_score": {
"query": {
"match": {
"titles.title": "sec"
}
},
"functions": [
{
"exp": {
"titles.depth": {
"origin": 0,
"scale": 1
}
}
}
]
}
}
}
}
}
In this example, document 1 is scored higher because it has a title matching sec at depth 0, whereas document 2 only has a title matching sec at depth 1.
The nested datatype and query ensure that the function_score associates the matching title with its depth, and the function score exp prioritizes titles with lower depth.