Elasticsearch searchable synthetic fields - elasticsearch

Question

Provided that in a source document (JSON) exist a couple of fields named, a and b,
that are of type long, I would like to construct a synthetic field (e.g. c)
by concatenating the values of the previous fields with an underscore and
index it as keyword.
That is, I am looking into a feature that could be supported with an imaginary, partial, mapping like this:
...
"a": { "type": "long" },
"b": { "type": "long" },
"c": {
"type": "keyword"
"expression": "${a}_${b}"
},
...
NOTE: The mapping above was made up just for the sake of the example. It is NOT valid!
So what I am looking for, is if there is a feature in elasticsearch, a recipe or hint to support
this requirement. The field need not be registered in _source, just need to be searchable.

Answer 1

There are 2 steps to this -- a dynamic_mapping and an ingest_pipeline.
I'm assuming your field c is non-trivial so you may want to match that field in a dynamic template using a match and assign the keyword mapping to it:
PUT synthetic
{
"mappings": {
"dynamic_templates": [
{
"c_like_field": {
"match_mapping_type": "string",
"match": "c*",
"mapping": {
"type": "keyword"
}
}
}
],
"properties": {
"a": {
"type": "long"
},
"b": {
"type": "long"
}
}
}
}
Then you can set up a pipeline which'll concatenate your a & b:
PUT _ingest/pipeline/combined_ab
{
"description" : "Concatenates fields a & b",
"processors" : [
{
"set" : {
"field": "c",
"value": "{{_source.a}}_{{_source.b}}"
}
}
]
}
After ingesting a new doc (with the activated pipeline!)
POST synthetic/_doc?pipeline=combined_ab
{
"a": 531351351351,
"b": 251531313213
}
we're good to go:
GET synthetic/_search
yields
{
"a":531351351351,
"b":251531313213,
"c":"531351351351_251531313213"
}
Verify w/ GET synthetic/_mapping too.