Elasticsearch searchable synthetic fields

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.