Failed avg_bucket aggregation when buckets_path include nested bucket with same name as other bucket.

buckets_with_same_name.sh

curl -XDELETE 'localhost:9200/docs?pretty'


curl -XPUT 'localhost:9200/docs?pretty' -H 'Content-Type: application/json' -d'
{
  "settings": {
    "index": {
      "number_of_shards": 1,
      "number_of_replicas": 1
    }
  },
  "mapping": {
    "doc": {
      "_all": {
        "enabled": false
      },
      "properties": {
        "date": {
          "type": "date",
          "format": "yyyy-MM-dd"
        },
        "group": {
          "type": "integer"
        }
      }
    }
  }
}
'

curl -XPOST 'localhost:9200/docs/doc?pretty' -H 'Content-Type: application/json' -d'
{
  "date": "2017-01-01",
  "group": 3
}
'

curl -XPOST 'localhost:9200/docs/doc?pretty' -H 'Content-Type: application/json' -d'
{
  "date": "2017-02-01",
  "group": 4
}
'

# the following aggregation will cause Error:
curl -XPOST 'localhost:9200/docs/doc/_search?pretty' -H 'Content-Type: application/json' -d'
{
  "size": 0,
  "aggs": {
    "avg_monthly": {
      "avg_bucket": {
        "buckets_path": "monthly.groups"
      }
    },
    "groups": {
      "cardinality": {
        "field": "group"
      }
    },
    "monthly": {
      "date_histogram": {
        "field": "date",
        "interval": "month"
      },
      "aggs": {
        "groups": {
          "cardinality": {
            "field": "group"
          }
        }
      }
    }
  }
}
'
# The error:
# {
#   "error" : {
#    "root_cause" : [ ],
#    "type" : "reduce_search_phase_exception",
#    "reason" : "[reduce] ",
#    "phase" : "merge",
#    "grouped" : true,
#    "failed_shards" : [ ],
#    "caused_by" : {
#      "type" : "class_cast_exception",
#      "reason" : "org.elasticsearch.search.aggregations.metrics.cardinality.InternalCardinality cannot be cast to org.elasticsearch.search.aggregations.InternalMultiBucketAggregation"
#    }
#  },
#  "status" : 503
# }


# when renaming the outer, not nested "groups" aggregation to "groups2" it works.
curl -XPOST 'localhost:9200/docs/doc/_search?pretty' -H 'Content-Type: application/json' -d'
{
  "size": 0,
  "aggs": {
    "avg_monthly": {
      "avg_bucket": {
        "buckets_path": "monthly.groups"
      }
    },
    "groups2": {
      "cardinality": {
        "field": "group"
      }
    },
    "monthly": {
      "date_histogram": {
        "field": "date",
        "interval": "month"
      },
      "aggs": {
        "groups": {
          "cardinality": {
            "field": "group"
          }
        }
      }
    }
  }
}
'

# result:
# {
#   "took" : 2,
#   "timed_out" : false,
#   "_shards" : {
#     "total" : 1,
#     "successful" : 1,
#     "failed" : 0
#   },
#   "hits" : {
#     "total" : 2,
#     "max_score" : 0.0,
#     "hits" : [ ]
#   },
#   "aggregations" : {
#     "groups2" : {
#       "value" : 2
#     },
#     "monthly" : {
#       "buckets" : [
#         {
#           "key_as_string" : "2017-01-01T00:00:00.000Z",
#           "key" : 1483228800000,
#           "doc_count" : 1,
#           "groups" : {
#             "value" : 1
#           }
#         },
#         {
#           "key_as_string" : "2017-02-01T00:00:00.000Z",
#           "key" : 1485907200000,
#           "doc_count" : 1,
#           "groups" : {
#             "value" : 1
#           }
#         }
#       ]
#     },
#     "avg_monthly" : {
#       "value" : 1.0
#     }
#   }
# }