[[painless-update-by-query-context]]
=== Update by query context

Use a Painless script in an
{ref}/docs-update-by-query.html[update by query] operation to add,
modify, or delete fields within each of a set of documents collected as the
result of query.

*Variables*

`params` (`Map`, read-only)::
        User-defined parameters passed in as part of the query.

`ctx['op']` (`String`)::
        The name of the operation.

{ref}/mapping-routing-field.html[`ctx['_routing']`] (`String`, read-only)::
        The value used to select a shard for document storage.

{ref}/mapping-index-field.html[`ctx['_index']`] (`String`, read-only)::
        The name of the index.

{ref}/mapping-type-field.html[`ctx['_type']`] (`String`, read-only)::
        The type of document within an index.

{ref}/mapping-id-field.html[`ctx['_id']`] (`int`, read-only)::
        The unique document id.

`ctx['_version']` (`int`, read-only)::
        The current version of the document.

{ref}/mapping-source-field.html[`ctx['_source']`] (`Map`)::
        Contains extracted JSON in a `Map` and `List` structure for the fields
        existing in a stored document.

*Side Effects*

`ctx['op']`::
        Use the default of `index` to update a document. Set to `none` to
        specify no operation or `delete` to delete the current document from
        the index.

{ref}/mapping-source-field.html[`ctx['_source']`]::
        Modify the values in the `Map/List` structure to add, modify, or delete
        the fields of a document.

*Return*

`void`::
        No expected return value.

*API*

The standard <<painless-api-reference, Painless API>> is available.

*Example*

To run this example, first follow the steps in
<<painless-context-examples, context examples>>.

The following query finds all seats in a specific section that have not been
sold and lowers the price by 2:

[source,console]
--------------------------------------------------
POST /seats/_update_by_query
{
  "query": {
    "bool": {
      "filter": [
        {
          "range": {
            "row": {
              "lte": 3
            }
          }
        },
        {
          "match": {
            "sold": false
          }
        }
      ]
    }
  },
  "script": {
    "source": "ctx._source.cost -= params.discount",
    "lang": "painless",
    "params": {
      "discount": 2
    }
  }
}
--------------------------------------------------
// TEST[setup:seats]
