title: Elasticsearch aggregations, get additional field in bucket
body: <p>I query ES index to filter results and get aggregations by selected terms. A sample query is like this:</p>
  
  <pre><code>GET buyer_requests/vehicle_requests/_search
  {
    "query": {
      "filtered": {
          "filter": {
            "and": [
              {
                "terms": {
                  "vehicle.make.raw": [
                    "Audi",
                    "BMW",
                    "Chevrolet"
                  ]
                }
              },
              {
                "range": {
                  "style.price": {
                    "gte": 15000,
                    "lte": 20000
                  }
                }
              },
              {
                "geo_distance": {
                  "distance": "20000km",
                  "info.pin": {
                    "lat": 42,
                    "lon": 21
                  }
                }
              }
            ]
          }
      }
    }, 
    "aggs": {
      "makes": {
        "filter": {
          "range": {
            "style.price": {
              "gte": 5000,
              "lte": 40000
            }
          }
        },
        "aggs": {
          "makes": {
            "terms": {
              "field": "vehicle.make.raw",
              "order": {
                "_term": "asc"
              }
            }
          }
        }
      },
      "model": {
        "filter": {
          "and": [
            {
              "terms": {
                "vehicle.make.raw": [
                  "Audi",
                  "BMW",
                  "Chevrolet"
                ]
              }
            }
          ]
        },
        "aggs": {
          "models": {
            "terms": {
              "field": "vehicle.model.raw",
              "size": 10,
              "order": {
                "_term": "asc"
              }
            }
          }
        }
      }
    }
  }
  </code></pre>
  
  <p>The result I get is something like:
  <img src="https://i.stack.imgur.com/5GyPC.jpg" alt="Elasicsearch results"></p>
  
  <p>How can I get in "buckets" section on "models" terms another field from result set. I want to get reference to Makes so the result would look like this:</p>
  
  <pre><code>"model": {
     "doc_count": 7,
     "models": {
        "doc_count_error_upper_bound": 0,
        "sum_other_doc_count": 0,
        "buckets": [
           {
              "key": "3 Series",
              "make": "bmw",                  &lt;----------- this key
              "doc_count": 3
           },
           {
              "key": "4 Series",
              "make": "bmw",                  &lt;----------- this key
              "doc_count": 4
           },
           {
             "key": "Camaro",
             "make": "chevrolet",             &lt;----------- this key
             "doc_count": 2
           }
        ]
     }
  }
  </code></pre>