Source filtering for first of nested objects

Question

I'm using source filtering on a query to just return a small set of information and within that information I want the first photo url of an object:

"query" : { ... },
"_source" : ["name", "photos.thumbnail_url"]

Of course this returns the thumbnail_url for all of the photos stored against each item.

Is there any way to get just the first nested photo's thumbnail_url using source filtering?

(I appreciate that one way to do it would be to index the first photo separately and just source filter to include that separate field but I'm asking whether it is possible without a separate field.)

(I've guessed a few approaches without luck: photos.0.thumbnail_url, photos[0].thumbnail_url, photos.first.thumbnail_url)

Have you tried inner_hits with size: 1?

Andrei Stefan
– Andrei Stefan

2015-10-09 10:26:48 +00:00
Commented Oct 9, 2015 at 10:26 — Andrei Stefan
– Andrei Stefan, Commented Oct 9, 2015 at 10:26

Andrei Stefan · Accepted Answer · 2015-10-09 11:05:47Z

2

Just an attempt, as I don't know your mapping, with inner_hits:

{
  "size": 5,
  "_source": ["name"], 
  "query": {
    "filtered": {
      "query": {
        "match": {
          "name": "whatever"
        }
      },
      "filter": {
        "nested": {
          "path": "photos",
          "query": {
            "match_all": {}
          },
          "inner_hits": {
            "size": 1,
            "_source": ["thumbnail_url"]
          }
        }
      }
    }
  }
}

or use a script field:

  "script_fields": {
    "FIELD": {
      "script": "urls=_source.photos.thumbnail_url;return urls[0]"
    }
  }

and I don't think it's possible to do this with source filtering.

answered Oct 9, 2015 at 11:05

Andrei Stefan

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7 Comments

Shadwell Over a year ago

Yep, that definitely returns just one photo thumbnail url. I can't seem to get it to return the first one though but I can index an, er, index field easily enough to sort on. Thanks for the help.

Andrei Stefan Over a year ago

The question is what does "first" mean to you? First as you indexed (like source first) or "first" by other definitions?

Shadwell Over a year ago

First as I indexed, yes, perhaps that just needs to be a bit more explicit.

Andrei Stefan Over a year ago

You tested with script_field or inner_hits?

Shadwell Over a year ago

I need to be a bit more explicit all round :) I tested with inner hits - my expectation is that script_field will return the first in the source but I've not got to testing that yet (have scripting off at the moment).

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