Finalize how search 'fields' option returns nested data.

[jtibshirani]

Currently the 'fields' option has no special handling for nested fields -- it just returns them in a flat list (as it does for a non-nested object array):

"fields": {
  "products.base_price": [43.99, 20.99],
  "products.manufacturer": ["Microlutions", "Elitelligence"],
  "products.product_id": [17426, 19288]
}

This drops the relationship between nested fields and could be misleading about the search behavior. Instead we could return an object array, where each entry contains the fields for a nested document:

"fields": {
  "products": [{
    "base_price": [43.99],
    "manufacturer": ["Microlutions"],
    "product_id": [17426],
  },
  {
    "base_price": [20.99],
    "manufacturer": ["Elitelligence"],
    "product_id": [19288],
  }]
}

Yet another option would be to not return any nested data when requesting 'fields' for the root doc, to match the behavior for docvalue_fields. Nested fields would only be available when using the inner_hits option. This is simple to implement but feels lacking: our users are familiar with _source filtering, which allows for accessing nested objects at the root level.

[elasticmachine]

Pinging @elastic/es-search (:Search/Search)

[astefan]

That's a difficult choice to make. I think I understand the reason behind returning nested documents in a more hierarchical way (one nested field might be missing a value, for example, and the array of values for that nested field doesn't shed any light into which nested document is the one that has the missing value) but at the same time one would argue that the format the nested documents are returned into is inconsistent with the one for the rest of the fields.

I'd say we could treat nested documents as special and keep the hierarchy, since they are indeed special. At the moment, with the current implementation, fields and _source extraction has a difference in it, so we could say that _source extraction and fields are not exactly the same anyway:
for a dep nested field and query

    "_source": "dep.dep*",
    "fields": [{"field": "dep.dep*"}]

the result is

                "_source": {
                    "dep": [
                        {
                            "dep_id": "d005",
                            "dep_name": "Development"
                        }
                    ]
                },
                "fields": {
                    "dep.dep_id": [
                        "d005"
                    ],
                    "dep.dep_name.keyword": [
                        "Development"
                    ],
                    "dep.dep_name": [
                        "Development"
                    ]
                }

In SQL I think we only return nested documents as part of inner_hits. Even if we ask for a nested field without any condition applied to it, we still build a nested query with a match_all in it and populate inner_hits accordingly.

[cbuescher]

With the asks of the Kibana team in Discover Switch from _source to fields when fetching data · Issue #80517 · elastic/kibana · GitHub in mind, I’m currently considering the following options:

• fetch all nested object paths that we recognise from the mapping service, get them from _source and add them to the fields API output. This shouldn’t collide with any other current output since we don’t currently add non-leave fields.
  • Option 1: just add the nested objects to the response, but leave current output of flattened subfields as is
    • advantages:
      • keeps information like e.g. keyword multi-fields of fields in the nested docs that are not part of _source
      • stays closer to the notion of the fields API to return a field centric view of the document
    • disadvantages: duplication of some fields content, might need pre-processing and filtering out on the client side
  • Option 2: remove all fields which have the nested path as prefix, e.g. if nested object is user, remove all additional user.* fields
    • advantages: potentially no additional filtering
    • disadvantages: might lose information like e.g. multi-fields under nested that are not in _source

Two unrelated decisions:

• Should we only return nested object closest to the document root, like asked for in [Discover] Switch from _source to fields when fetching data kibana#80517 (comment), or would it make sense to include all nested objects (e.g. alongside nester user, also consider returning separate user.adress when adress is another nested object under user.
  • advantages: I feel this would be more coherent since we’d return all nested objects that are accessible
  • disadvantages: larger response size
• Should all of this behaviour be hidden behind a flag?
  • this would have the advantage of keeping the “default” fields behaviour simpler and easier to explain, only clients that really need nested objects would need switch on this flag
  • I’d currently opt for a “global” flag that is directly defined under the main fields parameter in the request, e.g.

   “fields” : {
   	“include_nested” : true,  // would default to false
    	“fields” : [ “*” ] // maybe name this differently from the surrounding “fields”, e.g. “pattern” or “paths” ?
   }
  

[jtibshirani]

I’m currently considering the following options:

Are we also considering the option suggested in the issue description? I don't think this fits neatly into either option you listed. That option would still load all mapped content (including multi-fields), but make sure that the response captures the nested document structure.

[cbuescher]

Nested fields would only be available when using the inner_hits option.

@jtibshirani are you refering to this? I this came up in a discussion with Kibana we had lately, but the problem we saw with this is that using inner_hits requires knowledge of the nested paths and which objects are nested. While this information should theoretically be accesible I was under the impression that Kibana would rather like to query all fields (i.e the wildcard "*") and not have to inspect mappings upfront to know which parts might map to nested fields. Happy to discuss this again if I'm missing something or if this wasn't what you were refering to.

[jtibshirani]

Sorry for the confusion, I'm referring to the first option in the description where we return a response like this:

"fields": {
  "products": [{
    "base_price": [43.99],
    "manufacturer": ["Microlutions"],
    "product_id": [17426],
  },
  {
    "base_price": [20.99],
    "manufacturer": ["Elitelligence"],
    "product_id": [19288],
  }]
}

We separate out each nested document in the response, so the structure is preserved. Each nested section contains the 'fields' output, so multi-fields are included, formatting is applied, etc. There is no duplication of data -- for nested documents we only return this structured format instead of the flattened and combined fields.