Count child pages in Elasticsearch

If you do not have many items:

You can retrieve the information using only one query:

GET /test/page/_search{  "filter": {    "term": {      "Parent": "0"    }  },  "aggs": {    "numberOfChildren": {      "terms": {        "field": "Parent",        "size": 0      }    }  }}

In the response, hits.hits will contains the children of 0.

For each node you will have its number of children in aggregations.numberOfChildren.buckets with this structure:

{    "key": [page id],    "doc_count": [number of children for this page]}

Example response:

{  ...    "hits": {    "total": 1,    "max_score": 1,    "hits": [      {        "_index": "test",        "_type": "page",        "_id": "a",        "_score": 1,        "_source": {          "Id": "a",          "Parent": "0"        }      }    ]  },  "aggregations": {    "numberOfChildren": {      "doc_count_error_upper_bound": 0,      "sum_other_doc_count": 0,      "buckets": [        {          "key": "0",          "doc_count": 1        },        {          "key": "a",          "doc_count": 2        },        {          "key": "c",          "doc_count": 1        }      ]    }  }

Please not that:

• If the page does not have any hildren, it is not in the list.
• You have the number of children for all parents, not only the directchilds of 0, thus it will break if you have many items (too manybuckets).

If you have many items:

GET /test/page/_search{  "query": {    "filtered": {      "filter": {        "term": {          "Parent": "0"        }      }    }  }}

You will have 0's direct children in hits.hits.

GET /test/page/_search{  "size": 0,   "query": {    "filtered": {      "filter": {        "terms": {          "Parent": [            "a" // list 0's direct children ids          ]        }      }    }  },  "aggs": {    "numberOfChildren": {      "terms": {        "field": "Parent",        "size": 0,        "order": {          "_term": "asc"        }      }    }  }}

You will have the number of children of 0's direct children in aggregations.numberOfChildrens.buckets

You may also be able to use scripts, but I am not sure that they can work in this situation.

Parent-child relationship will not help you since parents and children cannot be of the same type.

I don't know if this is what you are searching for:

I've tried to insert same elements:

PUT /tmp_index/doc/1{    "id": "a",    "parent": "0"}PUT /tmp_index/doc/2{    "id": "b",    "parent": "a"}PUT /tmp_index/doc/3{    "id": "c",    "parent": "a"}PUT /tmp_index/doc/4{    "id": "d",    "parent": "c"}

With a nested aggregation like this:

POST /tmp_index/_search?pretty{   "size": 0,   "query": {      "match_all": {}   },   "aggs": {      "group_by_first": {         "terms": {            "field": "parent",             "size" : 0         },         "aggs": {            "group_by_second": {               "terms": {                  "field": "id",                  "size" : 0               }            }         }      }   }}

You get this result:

{           "key": "a",           "doc_count": 2,           "group_by_second": {              "doc_count_error_upper_bound": 0,              "sum_other_doc_count": 0,              "buckets": [                 {                    "key": "b",                    "doc_count": 1                 },                 {                    "key": "c",                    "doc_count": 1                 }              ]           }        },        {           "key": "0",           "doc_count": 1,           "group_by_second": {              "doc_count_error_upper_bound": 0,              "sum_other_doc_count": 0,              "buckets": [                 {                    "key": "a",                    "doc_count": 1                 }              ]           }        },        {           "key": "c",           "doc_count": 1,           "group_by_second": {              "doc_count_error_upper_bound": 0,              "sum_other_doc_count": 0,              "buckets": [                 {                    "key": "d",                    "doc_count": 1                 }              ]           }        }

[Updated]

I'm not sure if you can achieve it all in one query.

I needed something similar and ended up using msearch for the follow-up query.

POST /test/_msearch{}{"query" : {"term" : {"Parent": "c"}}}, "size" : 0}{}{"query" : {"term" : {"Parent": "d"}}}, "size" : 0}