# Meta-Fields（元字段） 每個文檔都有與之關聯的元數據，比如?`**_index**，``**[_type](https://www.elastic.co/guide/en/elasticsearch/reference/current/mapping-type-field.html "_type field")?**和?`**`_id?`**元字段。當創建?**mapping**?**type**（映射類型）時，可以自定義一些這些元字段的行為。 ### ?Identity meta-fields（身份的元字段） | [_index](https://www.elastic.co/guide/en/elasticsearch/reference/current/mapping-index-field.html) | 文檔所屬的索引。 | | [_uid](https://www.elastic.co/guide/en/elasticsearch/reference/current/mapping-uid-field.html) | 復合字段包含了**_type?**和?**_id**?。 | | [_type](https://www.elastic.co/guide/en/elasticsearch/reference/current/mapping-type-field.html) | 文檔的?[mapping type（映射類型 ）](https://www.elastic.co/guide/en/elasticsearch/reference/current/mapping.html#mapping-type)?。 | | [_id](https://www.elastic.co/guide/en/elasticsearch/reference/current/mapping-id-field.html) | 文檔 ID。 | ### ?Document source meta-fields（文檔源的元字段） [`_source`](https://www.elastic.co/guide/en/elasticsearch/reference/current/mapping-source-field.html "_source field") 原始的?**JSON?**字符串表示文檔的正文。 ?[`_size`](https://www.elastic.co/guide/en/elasticsearch/plugins/5.3/mapper-size.html) ?**_source?**字段的大小（以字節為單位），由?[mapper-size](https://www.elastic.co/guide/en/elasticsearch/plugins/5.3/mapper-size.html)?插件提供。 ### ?Indexing meta-fields（索引的元字段） [`_all`](https://www.elastic.co/guide/en/elasticsearch/reference/current/mapping-all-field.html "_all field") 一個索引其他全部字段值的全能字段。 ?[`_field_names`](https://www.elastic.co/guide/en/elasticsearch/reference/current/mapping-field-names-field.html "_field_names field") ?文檔中包含非空值的所有字段。 ### Routing meta-fields（路由的元字段） [`_parent`](https://www.elastic.co/guide/en/elasticsearch/reference/current/mapping-parent-field.html "_parent field") 用于創建兩個映射類型之間的父子關系。 [`_routing`](https://www.elastic.co/guide/en/elasticsearch/reference/current/mapping-routing-field.html "_routing field") 將文檔路由到特定分片的自定義路由值。 ### ?Other meta-field（其他的元字段） [`_meta`](https://www.elastic.co/guide/en/elasticsearch/reference/current/mapping-meta-field.html "_meta field") ?特定應用的元數據。