Is it possible to access a query term in a script field? - elasticsearch

Question

I would like to construct an elasticsearch query in which I can search for a term and on-the-fly compute a new field for each found document, which is calculated based on some existing fields as well as the query term. Is this possible?
For example, let's say in my EL query I am searching for documents which have the keyword "amsterdam" in the "text" field.
"filter": [
{
"match_phrase": {
"text": {
"query": "amsterdam"
}
}
}]
Now I would also like to have a script field in my query, which computes some value based on other fields as well as the query.
So far, I have only found how to access the other fields of a document though, using doc['someOtherField'], for example
"script_fields" : {
"new_field" : {
"script" : {
"lang": "painless",
"source": "if (doc['citizens'].value > 10000) {
return "large";
}
return "small";"
}
}
}
How can I integrate the query term, e.g. if I wanted to add to the if statement "if the query term starts with a-e"?

Answer 1

You're on the right track but script_fields are primarily used to post-process your documents' attributes — they won't help you filter any docs because they're run after the query phase.
With that being said, you can use scripts to filter your documents through script queries. Before you do that, though, you should explore alternatives.
In other words, scripts should be used when all other mechanisms and techniques have been exhausted.
Back to your example. I see three possibilities off the top of my head.
Match phrase prefix queries as a group of bool-should subqueries:
POST your-index/_search
{
"query": {
"bool": {
"must": [
{
"bool": {
"should": [
{
"match_phrase_prefix": {
"text_field": "a"
}
},
{
"match_phrase_prefix": {
"text_field": "b"
}
},
{
"match_phrase_prefix": {
"text_field": "c"
}
},
... till the letter "e"
]
}
}
]
}
}
}
A regexp query:
POST your-index/_search
{
"query": {
"bool": {
"must": [
{
"regexp": {
"text_field": "[a-e].+"
}
}
]
}
}
}
Script queries using .charAt comparisons:
POST your-index/_search
{
"query": {
"bool": {
"must": [
{
"script": {
"script": {
"source": """
char c = doc['text_field.keyword'].value.charAt(0);
return c >= params.gte.charAt(0) && c <= params.lte.charAt(0);
""",
"params": {
"gte": "a",
"lte": "e"
}
}
}
}
]
}
}
}
If you're relatively new to ES and would love to see real-world examples, check out my recently released Elasticsearch Handbook. One chapter is dedicated to scripting and as it turns out, you can achieve a lot with scripts (if of course executed properly).