# Return per-field similarity for each of many fields

**URL:** <https://discuss.elastic.co/t/return-per-field-similarity-for-each-of-many-fields/183254>\
**Category:** Elasticsearch\
**Created:** [May 29, 2019, 7:28am UTC](https://discuss.elastic.co/t/return-per-field-similarity-for-each-of-many-fields/183254 "2019-05-29T07:28:06Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![jeff-wellio](https://sea2.discourse-cdn.com/elastic/user_avatar/discuss.elastic.co/jeff-wellio/32/47040_2.png) [@jeff-wellio](https://discuss.elastic.co/u/jeff-wellio)\
**Post date:** [May 29, 2019, 7:28am UTC](https://discuss.elastic.co/t/return-per-field-similarity-for-each-of-many-fields/183254/1 "2019-05-29T07:28:06Z")

</div>

**Context:**  
We've implemented a learning-to-rank re-scorer for Elasticsearch hits, but (unfortunately) it has to live outside of Elasticsearch, i.e. as a microservice that takes candidates from Elasticsearch, re-scores each hit with a statistical model, and re-ranks those candidates by the model's scores before returning them to the user.

As features, the model uses fields from the document hit, from the query, and the `_score`, which sort of captures the combination of these.

**Goal:**  
I'd like to extend the statistical model to include the similarity between the query and each of a handful of individual document fields, as separate features. For example, I'd like Elasticsearch to include in the results of a `/_search`, some calculated fields such as all of these:

- `_score` (I use this already, but it's pretty coarse resolution)
- BM25 between query and document title
- BM25 between query and document url
- BM25 between query and document tags

and so on. I'm not picky, if I can't get BM25 exactly, some approximate value that roughly correlates is fine.

Ideally I need to get Elasticsearch to calculate these values and simply return them to me in the response payload. I _don't_ need it to change how Elasticsearch internally computes the `_score`.

**What I've tried**

I know that `?explain=true` interleaves these calculations directly into my hits, which in theory is all I need. The downside is that it bloats the payload at least 50x (it's sending far more than the few pieces of data I do need), and also it's a pain to programmatically parse out what I need from the explain report.

I think more useful would be to specify `script_fields`, one for each of the per-field BM25s I want back. Pseudocode:

```
GET /_search
{
    "query" : {
        "query_string": {
            "query": "purple island",
            "fields": ["title", "url", "description", "tags"]
        }
    },
    "script_fields" : {
        "sim_to_title" : {
            "script" : {
                "source": "doc['title'].PerFieldSimilarity"
            }
        },
        "sim_to_url" : {
            "script" : {
                "source": "doc['url'].PerFieldSimilarity"
            }
        }
    }
}

```

but I'm at a loss as to what these scripts actually need to look like. I found [https://www.elastic.co/guide/en/elasticsearch/painless/6.5/painless-similarity-context.html](https://www.elastic.co/guide/en/elasticsearch/painless/6.5/painless-similarity-context.html) but not fully sure how to use it; I get the feeling these only work at index time to define a custom similarity metric, whereas I need to return all these values at search time.

What ideas does this community have?

Thanks very much! --Jeff

---

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**Author:** ![mayya](https://sea2.discourse-cdn.com/elastic/user_avatar/discuss.elastic.co/mayya/32/83147_2.png) [@mayya](https://discuss.elastic.co/u/mayya)\
**Post date:** [June 4, 2019, 10:04am UTC](https://discuss.elastic.co/t/return-per-field-similarity-for-each-of-many-fields/183254/2 "2019-06-04T10:04:52Z")

</div>

There is no way to produce scores through `script_fields`. Painless has [different contexts](https://www.elastic.co/guide/en/elasticsearch/painless/master/painless-contexts.html) and in the [field context](https://www.elastic.co/guide/en/elasticsearch/painless/master/painless-field-context.html) we can only have access to the values of document's fields.

Moreover, each search request will produce scores in a single way. The only way to achieve what you are trying to do is to issue 4 different search requests, 3 of which will be [multi match](https://www.elastic.co/guide/en/elasticsearch/reference/current/query-dsl-multi-match-query.html) queries.

We also currently working on [improving scoring across multiple fields](https://github.com/elastic/elasticsearch/issues/41106) which can also take document statistics into account, but this is work in progress.

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<div class="post-metadata">

**Author:** ![jeff-wellio](https://sea2.discourse-cdn.com/elastic/user_avatar/discuss.elastic.co/jeff-wellio/32/47040_2.png) [@jeff-wellio](https://discuss.elastic.co/u/jeff-wellio)\
**Post date:** [June 10, 2019, 10:26pm UTC](https://discuss.elastic.co/t/return-per-field-similarity-for-each-of-many-fields/183254/3 "2019-06-10T22:26:43Z")

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Thanks for this information! In particular it was helpful to know that `script_fields` happens in a Painless Field context; I'd seen those contexts listed but the documentation is missing the mapping between context and other ES syntax such as `script_fields`.

Regarding the "improving scoring across multiple fields" issue... I don't think adds much to my use case, at least in the sense that our machine learning model re-ranks results according to specific combinations of fields, a method that goes beyond the heuristics of BM25/BM25F.

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<div class="post-metadata">

**Author:** ![system](https://us1.discourse-cdn.com/elastic/original/3X/1/a/1ac57faf039f6b580b3f104ef42a2a89e41014de.png) [@system](https://discuss.elastic.co/u/system)\
**Post date:** [July 8, 2019, 10:26pm UTC](https://discuss.elastic.co/t/return-per-field-similarity-for-each-of-many-fields/183254/4 "2019-07-08T22:26:45Z")

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