array-of-objects.md

Array of objects and nested field type

How arrays of objects are flattened

Elasticsearch has no concept of inner objects. Therefore, it flattens object hierarchies into a simple list of field names and values. For instance, consider the following document:

PUT my-index-000001/_doc/1
{
  "group" : "fans",
  "user" : [
    {
      "first" : "John",
      "last" :  "Smith"
    },
    {
      "first" : "Alice",
      "last" :  "White"
    }
  ]
}

The user field is dynamically added as a field of type object.

This document would be transformed internally into a document that looks more like this:

{
  "group" :        "fans",
  "user.first" : [ "alice", "john" ],
  "user.last" :  [ "smith", "white" ]
}

The user.first and user.last fields are flattened into multi-value fields, and the association between alice and white is lost. This document would incorrectly match a query for alice AND smith:

GET my-index-000001/_search
{
  "query": {
    "bool": {
      "must": [
        { "match": { "user.first": "Alice" }},
        { "match": { "user.last":  "Smith" }}
      ]
    }
  }
}

Result:

{
  "took" : 7,
  "timed_out" : false,
  "_shards" : {
    "total" : 1,
    "successful" : 1,
    "skipped" : 0,
    "failed" : 0
  },
  "hits" : {
    "total" : {
      "value" : 1,
      "relation" : "eq"
    },
    "max_score" : 0.5753642,
    "hits" : [
      {
        "_index" : "my-index-000001",
        "_type" : "_doc",
        "_id" : "1",
        "_score" : 0.5753642,
        "_source" : {
          "group" : "fans",
          "user" : [
            {
              "first" : "John",
              "last" : "Smith"  
            },
            {
              "first" : "Alice",
              "last" : "White"
            }
          ]
        }
      }
    ]
  }
}

Using nested field type for arrays of objects

If you need to index arrays of objects and to maintain the independence of each object in the array, use the nested data type instead of the object data type.

Internally, nested objects index each object in the array as a separate hidden document, meaning that each nested object can be queried independently of the others with the nested query:

The user field is mapped as type nested instead of type object.

PUT my-index-000001
{
  "mappings": {
    "properties": {
      "user": {
        "type": "nested"
      }
    }
  }
}

PUT my-index-000001/_doc/1
{
  "group" : "fans",
  "user" : [
    {
      "first" : "John",
      "last" :  "Smith"
    },
    {
      "first" : "Alice",
      "last" :  "White"
    }
  ]
}

This query doesn’t match because Alice and Smith are not in the same nested object.

GET my-index-000001/_search
{
  "query": {
    "nested": {
      "path": "user",
      "query": {
        "bool": {
          "must": [
            { "match": { "user.first": "Alice" }},
            { "match": { "user.last":  "Smith" }}
          ]
        }
      }
    }
  }
}

This query matches because Alice and White are in the same nested object. inner_hits allow us to highlight the matching nested documents.

GET my-index-000001/_search
{
  "query": {
    "nested": {
      "path": "user",
      "query": {
        "bool": {
          "must": [
            { "match": { "user.first": "Alice" }},
            { "match": { "user.last":  "White" }}
          ]
        }
      },
      "inner_hits": {
        "highlight": {
          "fields": {
            "user.first": {}
          }
        }
      }
    }
  }
}

https://opster.com/guides/elasticsearch/data-structuring/elasticsearch-nested-field-object-field/