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