Conditional sorting of hits in Elasticsearch - elasticsearch

Question

I've a index with road names. My settings look like this:
"settings": {
"max_ngram_diff": 20,
"analysis": {
"analyzer": {
"str_search_analyzer": {
"type": "custom",
"tokenizer": "whitespace",
"filter": [
"lowercase"
]
},
"str_index_analyzer": {
"type": "custom",
"tokenizer": "whitespace",
"filter": [
"lowercase",
"substring"
]
}
},
"filter": {
"substring": {
"type": "edgeNGram",
"min_gram": 1,
"max_gram": 255
}
}
}
}
}
In the index I've strings like this:
Bar Road
Bar Foo Road
Foo Road
So when I search for 'Foo' I get #2 and #3 as hits. This is expected.
But I would like to control the order of the hits. In this case I would like to have #3 as first hit, because the string starts with the search term.
Is it possible to sort hits as I want?

Answer 1

The following is based on this answer but has been adapted to your use case.
PUT sorting
{
"mappings": {
"properties": {
"text": {
"type": "text",
"fields": {
"analyzed": {
"type": "text",
"analyzer": "str_index_analyzer",
"search_analyzer": "str_search_analyzer",
"fielddata": true
},
"keyword": {
"type": "keyword"
}
}
}
}
},
"settings": {
"max_ngram_diff": 20,
"analysis": {
"analyzer": {
"str_search_analyzer": {
"type": "custom",
"tokenizer": "whitespace",
"filter": [
"lowercase"
]
},
"str_index_analyzer": {
"type": "custom",
"tokenizer": "whitespace",
"filter": [
"lowercase",
"substring"
]
}
},
"filter": {
"substring": {
"type": "edgeNGram",
"min_gram": 1,
"max_gram": 255
}
}
}
}
}
GET sorting/_search
{
"query": {
"function_score": {
"query": {
"match": {
"text.analyzed": "Foo"
}
},
"functions": [
{
"script_score": {
"script": {
"source": """
def docval = doc['text.keyword'].value;
def length = docval.length();
def index = (float) docval.indexOf('Foo');
// the sooner the word appears the better so 'invert' the 'index'
return index > -1 ? (1 / index) : 0;
"""
}
}
}
],
"boost_mode": "sum"
}
}
}