For unsharded collections this contains a single field _id.对于未排序的集合，它包含一个字段_id。 For sharded collections, this will contain all the components of the shard key对于分片集合，它将包含分片键的所有组件
_id
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For unsharded collections this contains a single field对于未排序的集合，它包含一个字段_id._id。For sharded collections, this will contain all the components of the shard key对于分片集合，它将包含分片键的所有组件