Elasticsearch sort by single nested document key in array

Question

I have documents which look like this (here are two examples):

{
    "id": 1234,
    "title": "the title",
    "body": "the body",
    "examples": [
        {
            "evidence_source": "friend",
            "source_score": 15
        },
        {
            "evidence_source": "parent",
            "source_score": 12
        }
    ]
}

and

{
    "id": 6346,
    "title": "new title",
    "body": "lots of content",
    "examples": [
        {
            "evidence_source": "friend",
            "source_score": 10
        },
        {
            "evidence_source": "parent",
            "source_score": 27
        },
        {
            "evidence_source": "child",
            "source_score": 4
        }
    ]
}

The format of the sub-documents in the examples array will always have an evidence_source and a source_score but there will be a variable amount of these sub-documents, each with different evidence_source values.

I am wondering if it is possible to sort documents with this format based on one of the source_score values matched to a specific evidence_source value. I'd really like to be able to do this:

• Sort documents by source_score descending where the related evidence_source is friend. The resulting ordering of the document ids would be 1234,6346.
• Sort documents by source_score descending where the related evidence_source is parent. The resulting ordering of the document ids would be 6346,1234.

The closest results that I'm come up with for doing something like this are 1 and 2 but I don't believe that they get at exactly what I want to do.

Any ideas about how I might go about this? I've contemplated some ideas based on indexing these examples sub-documents separately, but I'm fairly new to elasticsearch and so am looking for some advice on a how to achieve my goal in the most straightforward manner (which may be a pipe-dream...)

Update: A post on the elasticsearch mailing list seems to indicate that this is NOT possible, but I'm wondering if someone else here has any different ideas!

Answer 1

Support for sorting based on fields inside of nested documents was added to elasticsearch in 0.90:

https://github.com/elasticsearch/elasticsearch/issues/2662

The sorting by nested field support has the following parameters on top of the already existing sort options:

• nested_path - Defines the on what nested object to sort. The actual sort field must be a direct field inside this nested object. The default is to use the most immediate inherited nested object from the sort field.
• nested_filter - A filter the inner objects inside the nested path should match with in order for its field values to be taken into account by sorting. Common case is to repeat the query / filter inside the nested filter or query. By default no nested_filter is active.

Given your example data, the following query should give you what you're after:

{
  "query": {
    "match_all": {}
  },
  "sort": [
    {
      "examples.source_score": {
        "order": "desc",
        "nested_path": "examples",
        "nested_filter": {
          "term": {
            "examples.evidence_source": "friend"
          }
        }
      }
    }
  ]
}