[[query-dsl-has-child-filter]]
=== Has Child Filter

The `has_child` filter accepts a query and the child type to run
against, and results in parent documents that have child docs matching
the query. Here is an example:

[source,js]
--------------------------------------------------
{
    "has_child" : {
        "type" : "blog_tag",
        "query" : {
            "term" : {
                "tag" : "something"
            }
        }
    }
}    
--------------------------------------------------

The `type` is the child type to query against. The parent type to return
is automatically detected based on the mappings.

The way that the filter is implemented is by first running the child
query, doing the matching up to the parent doc for each document
matched.

The `has_child` filter also accepts a filter instead of a query:

[source,js]
--------------------------------------------------
{
    "has_child" : {
        "type" : "comment",
        "filter" : {
            "term" : {
                "user" : "john"
            }
        }
    }
}    
--------------------------------------------------

[float]
==== Memory Considerations

With the current implementation, all `_id` values are loaded to memory
(heap) in order to support fast lookups, so make sure there is enough
memory for it.

[float]
==== Caching

The `has_child` filter cannot be cached in the filter cache. The `_cache`
and `_cache_key` options are a no-op in this filter. Also any filter that
wraps the `has_child` filter either directly or indirectly will not be cached.