Nested inner hits in FreeText search use case - json

Question

I building a standard free text search on a site that sells cars.
In the search box the user can enter a search word that are passed on to the query where it is used to match both nested and non-nested properties.
I'm using inner_hits to limit the number of variants returned by the query (in this sample variants is not remove from _source)
When matching on a nested property color the inner_hits collection contains the correct variant as expected.
However when matching on a non-nested property title the inner_hits collection is empty. I understand why it's empty.
Can you suggest a better way to structure the query?
Another option would be to always just return at least 1 variant - but how can the be achieved?
Mappings
PUT test
{
"mappings": {
"car": {
"properties": {
"variants": {
"type": "nested"
}
}
}
}
}
Insert data
PUT test/car/1
{
"title": "VW Golf",
"variants": [
{
"color": "red",
"forsale": true
},
{
"color": "blue",
"forsale": false
}
]
}
Query by color
GET test/_search
{
"query": {
"nested": {
"path": "variants",
"query": {
"match": {
"variants.color": "blue"
}
},
"inner_hits": {}
}
}
}
Color query: works as expected!
"hits" : [
{
"_source" : {
"title" : "VW Golf",
"variants" : [
{
"color" : "red",
"forsale" : true
},
{
"color" : "blue",
"forsale" : false
}
]
},
"inner_hits" : {
"variants" : {
"hits" : {
"total" : 1,
"hits" : [
{
"_nested" : {
"field" : "variants",
"offset" : 1
},
"_source" : {
"color" : "blue",
"forsale" : false
}
}
]
}
}
}
}
]
Query by brand
GET test/_search
{
"query": {
"bool": {
"should": [
{
"match": {
"title": "golf"
}
},
{
"nested": {
"path": "variants",
"query": {
"match": {
"variants.color": "golf"
}
},
"inner_hits": {}
}
}
]
}
}
}
Brand query result :-(
"hits" : [
{
"_source" : {
"title" : "VW Golf",
"variants" : [
{
"color" : "red",
"forsale" : true
},
{
"color" : "blue",
"forsale" : false
}
]
},
"inner_hits" : {
"variants" : {
"hits" : {
"total" : 0,
"hits" : [ ]
}
}
}
}

Answer 1

You already know it but inner_hits returns an empty array because no nested documents matched in the nested query.
A simple solution is to change the query such that the nested query will always match. This can be done by wrapping the nested query into a bool query and add a match_all query.
If you set the boost of the match_all query to 0, it will not contribute to the score. Consequently, if a nested document match it will be first.
Now the inner hits will not be empty, but there is a second problem, all the documents will match. You can either:
set a min_score with a very small value (e.g., 0.00000001) to discard document with a score of 0
duplicate the original nested query and use a minimum_should_match at 2.
{
"query": {
"bool": {
// Ensure that at least 1 of the first 2 queries will match
// The third query will always match
"minimum_should_match": 2,
"should": [
{
"match": {
"title": <SEARCH_TERM>
}
},
{
"nested": {
"path": "variants",
"query": {
"match": {
"variants.color": <SEARCH_TERM>
}
}
}
},
{
"nested": {
"path": "variants",
"query": {
"bool": {
"should": [
{
"match": {
"variants.color": <SEARCH_TERM>
}
},
{
// Disable scoring
"match_all": { "boost": 0 }
}
]
}
},
"inner_hits": {}
}
}
]
}
}
}

Answer 2

One way to do it is using a script_fields clause.
You would write a little script in painless that would do the following:
store the List you get from variants in a variable
then iterate over the Maps in this List
if the Map has
color blue you return the Map . (If none evaluate to true you return an empty
Map). This would create an additional field per searchresult with only those variants where the color is blue.
One important drawback is that this is a very heavy operation, especially if you have many records.
You can take this approach if it is something only you will ever do, maybe a few times a year outside peak hours. If your use case is something with regular use and to be performed by many users, I would change the mapping, return variants as a whole or choose some other solution.