how to return the count of unique documents by using elasticsearch aggregation

Question

I encountered a problem that elasticsearch could not return the count of unique documents by just using terms aggregation on a nested field.

Here is an example of ou

Answer 1

I think you need a reverse_nested aggregation, because you want aggregation based on a nested value, but actually counting the ROOT documents, not the nested ones

{
  "query": {
    "bool": {
      "must": [
        {
          "term": {
            "last_name": "smith"
          }
        }
      ]
    }
  },
  "aggs": {
    "location": {
      "nested": {
        "path": "location"
      },
      "aggs": {
        "state": {
          "terms": {
            "field": "location.state",
            "size": 10
          },
          "aggs": {
            "top_reverse_nested": {
              "reverse_nested": {}
            }
          }
        }
      }
    }
  }
}

And, as a result, you would see something like this:

"aggregations": {
      "location": {
         "doc_count": 6,
         "state": {
            "doc_count_error_upper_bound": 0,
            "sum_other_doc_count": 0,
            "buckets": [
               {
                  "key": "ny",
                  "doc_count": 4,
                  "top_reverse_nested": {
                     "doc_count": 2
                  }
               },
               {
                  "key": "ca",
                  "doc_count": 2,
                  "top_reverse_nested": {
                     "doc_count": 2
                  }
               }
            ]
         }
      }
   }

And what you are looking for is under top_reverse_nested part. One point here: if I'm not mistaking "doc_count": 6 is the NESTED document count, so don't be confused about these numbers thinking you are counting root documents, the count is on the nested ones. So, for a document with three nested ones that match, the count would be 3, not 1.