[[mapping-id-field]]
=== `_id`

Each document indexed is associated with an id and a type. The `_id`
field can be used to index just the id, and possible also store it. By
default it is not indexed and not stored (thus, not created).

Note, even though the `_id` is not indexed, all the APIs still work
(since they work with the `_uid` field), as well as fetching by ids
using `term`, `terms` or `prefix` queries/filters (including the
specific `ids` query/filter).

The `_id` field can be enabled to be indexed, and possibly stored,
using the appropriate mapping attributes:

[source,js]
--------------------------------------------------
{
    "tweet" : {
        "_id" : {
            "index" : "not_analyzed",
            "store" : true
        }
    }
}
--------------------------------------------------

The `_id` mapping can also be associated with a `path` that will be used
to extract the id from a different location in the source document. For
example, having the following mapping:

[source,js]
--------------------------------------------------
{
    "tweet" : {
        "_id" : {
            "path" : "post_id"
        }
    }
}
--------------------------------------------------

Will cause `1` to be used as the id for:

[source,js]
--------------------------------------------------
{
    "message" : "You know, for Search",
    "post_id" : "1"
}
--------------------------------------------------

This does require an additional lightweight parsing step while indexing,
in order to extract the id to decide which shard the index operation
will be executed on.