# Children Aggregation

一种特殊的单桶聚合，可以将父文档类型上的桶聚合到子文档上。

此聚合依赖于映射中的[\_parent](https://www.elastic.co/guide/en/elasticsearch/reference/current/mapping-parent-field.html)字段。 此聚合有一个选项：

* type - 应该映射到父空间中的子类型的buckets(桶)

例如，假设我们有一个问题和答案的索引。 答案类型在映射中具有以下\_parent字段

```
PUT child_example
{
    "mappings": {
        "answer" : {
            "_parent" : {
                "type" : "question"
            }
        }
    }
}
```

问题类型的文档包含标签字段，答案类型的文档包含所有者字段。 使用子集合，即使两个字段存在于两种不同类型的文档中，标记桶也可以在单个请求中映射到所有者桶。

一个问题类型文档的例子

```
PUT child_example/question/1
{
    "body": "<p>I have Windows 2003 server and i bought a new Windows 2008 server...",
    "title": "Whats the best way to file transfer my site from server to a newer one?",
    "tags": [
        "windows-server-2003",
        "windows-server-2008",
        "file-transfer"
    ]
}
```

回答类型文件的例子:

```
PUT child_example/answer/1?parent=1&refresh
{
    "owner": {
        "location": "Norfolk, United Kingdom",
        "display_name": "Sam",
        "id": 48
    },
    "body": "<p>Unfortunately you're pretty much limited to FTP...",
    "creation_date": "2009-05-04T13:45:37.030"
}
PUT child_example/answer/2?parent=1&refresh
{
    "owner": {
        "location": "Norfolk, United Kingdom",
        "display_name": "Troll",
        "id": 49
    },
    "body": "<p>Use Linux...",
    "creation_date": "2009-05-05T13:45:37.030"
}
```

可以构建将两者连接在一起的以下请求：

```
POST child_example/_search?size=0
{
  "aggs": {
    "top-tags": {
      "terms": {
        "field": "tags.keyword",
        "size": 10
      },
      "aggs": {
        "to-answers": {
          "children": {
            "type" : "answer"  ＃1
          },
          "aggs": {
            "top-names": {
              "terms": {
                "field": "owner.display_name.keyword",
                "size": 10
              }
            }
          }
        }
      }
    }
  }
}
```

| 1 | type 指向使用名称为answer的 type / mapping |
| - | ---------------------------------- |

以上示例返回顶级问题标签和每个标签的顶级答案所有者。

下面是可能的返回结果：

```
{
  "timed_out": false,
  "took": 25,
  "_shards": { "total": 5, "successful": 5, "failed": 0 },
  "hits": { "total": 3, "max_score": 0.0, hits: [] },
  "aggregations": {
    "top-tags": {
      "doc_count_error_upper_bound": 0,
      "sum_other_doc_count": 0,
      "buckets": [
        {
          "key": "file-transfer",
          "doc_count": 1, ＃1
          "to-answers": {
            "doc_count": 2, ＃2
            "top-names": {
              "doc_count_error_upper_bound": 0,
              "sum_other_doc_count": 0,
              "buckets": [
                {
                  "key": "Sam",
                  "doc_count": 1
                },
                {
                  "key": "Troll",
                  "doc_count": 1
                }
              ]
            }
          }
        },
        {
          "key": "windows-server-2003",
          "doc_count": 1, ＃3
          "to-answers": {
            "doc_count": 2, ＃4
            "top-names": {
              "doc_count_error_upper_bound": 0,
              "sum_other_doc_count": 0,
              "buckets": [
                {
                  "key": "Sam",
                  "doc_count": 1
                },
                {
                  "key": "Troll",
                  "doc_count": 1
                }
              ]
            }
          }
        },
        {
          "key": "windows-server-2008",
          "doc_count": 1, ＃5
          "to-answers": {
            "doc_count": 2, ＃6
            "top-names": {
              "doc_count_error_upper_bound": 0,
              "sum_other_doc_count": 0,
              "buckets": [
                {
                  "key": "Sam",
                  "doc_count": 1
                },
                {
                  "key": "Troll",
                  "doc_count": 1
                }
              ]
            }
          }
        }
      ]
    }
  }
}
```

＃1 带有file-transfer，windows-server-2003等标签问题文档数量

＃3

＃5

＃2 带有`file-transfer`, `windows-server-2003等标签的与问题文档相关的答复文档的数量`

＃4

＃


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