# Meta-Fields（元字段） 每个文档都有与之关联的元数据，比如 `**_index**，``**[_type](https://www.elastic.co/guide/en/elasticsearch/reference/current/mapping-type-field.html "_type field") **和 `**`_id `**元字段。当创建 **mapping** **type**（映射类型）时，可以自定义一些这些元字段的行为。 ###  Identity meta-fields（身份的元字段） | [_index](https://www.elastic.co/guide/en/elasticsearch/reference/current/mapping-index-field.html) | 文档所属的索引。 | | [_uid](https://www.elastic.co/guide/en/elasticsearch/reference/current/mapping-uid-field.html) | 复合字段包含了**_type **和 **_id** 。 | | [_type](https://www.elastic.co/guide/en/elasticsearch/reference/current/mapping-type-field.html) | 文档的 [mapping type（映射类型 ）](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) | 文档 ID。 | ###  Document source meta-fields（文档源的元字段） [`_source`](https://www.elastic.co/guide/en/elasticsearch/reference/current/mapping-source-field.html "_source field") 原始的 **JSON **字符串表示文档的正文。  [`_size`](https://www.elastic.co/guide/en/elasticsearch/plugins/5.3/mapper-size.html)  **_source **字段的大小（以字节为单位），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") 一个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")  文档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") 用于创建两个映射类型之间的父子关系。 [`_routing`](https://www.elastic.co/guide/en/elasticsearch/reference/current/mapping-routing-field.html "_routing field") 将文档路由到特定分片的自定义路由值。 ###  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")  特定应用的元数据。