composite aggregation as child aggregation

[flefebure]

The 6.x new composite aggregation provides a way to scroll and page on terms bucket, and it's a good thing.
However, it's not (for now?) possible to set a composite aggregation as a child of another bucket aggregation.
So it's impossible, for example, to composite-aggregate on a nested or a child entity. This request is not allowed :

{
        "metadatas" : {
            "nested" : {
                "path" : "metaDatas"
            },
            "aggs" : {
                "msisdn" : {
                  "filter" : { "term": { "metaDatas.name": "msisdn" } },
                  "aggs": {
                    "msisdns": {
                       "composite" : {
                          "size":5,
                          "sources" : [{ "value": { "terms" : { "field": "metaDatas.value.raw" } } }]
                       }
                    }
                  }
                }
            }
        }
    }

I don't understand this limitation.
Furthermore, an ES 6.2 patched without the check seems to work as expected.

@Override
   protected AggregatorFactory<?> doBuild(SearchContext context, AggregatorFactory<?> parent,
                                          AggregatorFactories.Builder subfactoriesBuilder) throws IOException {
       /*if (parent != null) {
           throw new IllegalArgumentException("[composite] aggregation cannot be used with a parent aggregation");
       }*/

any insight ?

[jimczi]

The limitation was added because the use case was not clear but it is possible as long as all* sources in the composite are in the same nested context. I'll mark this issue with the feature label but as a low priority for now.

[colings86]

@elastic/es-search-aggs

[tchiotludo]

This feature is really useful, since nested is also not allow in sources of composite.
With out this features, we just can't use composite with nested document and must use deprecated terms aggregations with size = Infinity

[neelam28m]

I also feel this should be possible to have composite aggregation for nested object. I have come across a situation where sources with nested object is the only option and that too does not support nested types. I will have to look for other workaround, may be to use multiple queries to get the desired result.

[rooboo]

I find this feature also very helpful to get all values of a specific field in a nested object, grouped by an identifier.

@jimczi Can you possibly provide an example of how to write the composite aggregation when "all* sources in the composite are in the same nested context"? I'm trying to do this, but so far have not been successful.

[jimczi]

@justinmcp88 this is not possible currently which is why this issue is still open. I mentioned the all fields the composite must be in the same nested context as a way to implement the feature described in this issue.