fetch_fields.md

Fetch Fields

The fetch_fields option can be used to return fields next to the highlighted field. It is designed for use with object fields but has a number of limitations. For example, while highlighting the foo field in the document below it can be used to fetch the "bar" field:

{
  "test": [
    { "foo": "skipped", "bar": 1 },
    { "foo": "matched", "bar": 2 }
  ]
}

Here is the query:

{
  "_source": false,
  "query": {
    "term": {
      "test.foo": "matched"
    }
  },
  "highlight": {
    "fields": {
      "test.foo": {
        "type": "experimental",
        "options": {
          "fetch_fields": [ "test.bar" ]
        }
      }
    }
  }
}

And here is the interesting part of the output:

  "highlight" : {
    "foo" : [ "<em>matched</em>", "2" ]
  }

Notice that 2 is returned as a string and just looks like another segment. This return format is a compromise with the way Elasticsearch expects segments to be returned. It doesn't count against the segment limit. It just looks a bit funny.

But this document won't work:

{
  "test": [
    { "foo": "skipped" },
    { "foo": "matched", "bar": 2 }
  ]
}

The thing is, fetch_fields uses a trick to pick the right field to fetch: in most documents the ordinal positions of all the multi-valued fields line up regardless of actual document structure. Meaning this document works just fine:

{
  "test": [
    { "foo": "skipped", "bar": "" },
    { "foo": "matched", "bar": 2 }
  ]
}

and so does this one:

{
  "test": [
    { "foo": ["skipped", "matched"], "bar": [1, 2] }
  ]
}

Use it but be careful.