ElasticSearch按数组第一个索引处的NestedObject中的字段排序

Question

I am trying to sort a field inside the first object of an array in the following docs each docs has an array i want to retrieve the docs sorted by they first objects by there city name lets name that in the following result I want to have first the third documents because the name of the city its start by "L" ('london') then the second "M" ('Moscow') then the third "N" ('NYC')

the structure is a record that:

1. has an array
2. the array contains an object (called 'address')
3. the object has a field (called 'city')

i want to sort the docs by the first address.cities

    get hello/_mapping 
    {
      "hello": {
        "mappings": {
          "jack": {
            "properties": {
              "houses": {
                "type": "nested",
                "properties": {
                  "address": {
                    "properties": {
                      "city": {
                        "type": "text",
                        "fields": {
                          "keyword": {
                            "type": "keyword",
                            "ignore_above": 256
                          }
                        }
                      }
                    }
                  }
                }
              }
            }
          }
        }
      }
    }

Thos are the document that i indexed

{
  "took": 0,
  "timed_out": false,
  "_shards": {
    "total": 5,
    "successful": 5,
    "failed": 0
  },
  "hits": {
    "total": 3,
    "max_score": 1,
    "hits": [
      {
        "_index": "hello",
        "_type": "jack",
        "_id": "2",
        "_score": 1,
        "_source": {
          "houses": [
            {
              "address": {
                "city": "moscow"
              }
            },
            {
              "address": {
                "city": "belgrade"
              }
            },
            {
              "address": {
                "city": "Sacramento"
              }
            }
          ]
        }
      },
      {
        "_index": "hello",
        "_type": "jack",
        "_id": "1",
        "_score": 1,
        "_source": {
          "houses": [
            {
              "address": {
                "city": "NYC"
              }
            },
            {
              "address": {
                "city": "PARIS"
              }
            },
            {
              "address": {
                "city": "TLV"
              }
            }
          ]
        }
      },
      {
        "_index": "hello",
        "_type": "jack",
        "_id": "3",
        "_score": 1,
        "_source": {
          "houses": [
            {
              "address": {
                "city": "London"
              }
            }
          ]
        }
      }
    ]
  }
}

Answer 1

Try this (of course, add some test inside the script if field could be empty. Note it could be pretty slow, because elastic wont have this value indexed. Add a main address field would be faster (and really faster) for sure and would be the good way to do it.

{
      "sort" : {
        "_script" : {
            "script" : "params._source.houses[0].address.city",
            "type" : "string",
            "order" : "asc"
        }
    }
}

You have to use _source instead of doc[yourfield] because you dont know in witch order elastic store your array.

EDIT: test if field exist

{
  "query": {
    "nested": {
      "path": "houses",
      "query": {
        "bool": {
          "must": [
            {
              "exists": {
                "field": "houses.address"
              }
            }
          ]
        }
      }
    }
  },
  "sort": {
    "_script": {
      "script" : "params._source.houses[0].address.city",
      "type": "string",
      "order": "asc"
    }
  }
}