[[query-dsl-has-child-query]]
== Has Child Query

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"
            }
        }
    }
}
--------------------------------------------------

[float]
=== Scoring capabilities

The `has_child` also has scoring support. The
supported score types are `min`, `max`, `sum`, `avg` or `none`. The default is
`none` and yields the same behaviour as in previous versions. If the
score type is set to another value than `none`, the scores of all the
matching child documents are aggregated into the associated parent
documents. The score type can be specified with the `score_mode` field
inside the `has_child` query:

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

[float]
=== Min/Max Children

The `has_child` query allows you to specify that a minimum and/or maximum
number of children are required to match for the parent doc to be considered
a match:

[source,js]
--------------------------------------------------
{
    "has_child" : {
        "type" : "blog_tag",
        "score_mode" : "sum",
        "min_children": 2, <1>
        "max_children": 10, <1>
        "query" : {
            "term" : {
                "tag" : "something"
            }
        }
    }
}
--------------------------------------------------
<1> Both `min_children` and `max_children` are optional.

The  `min_children` and `max_children` parameters can be combined with
the `score_mode` parameter.

[float]
=== Memory Considerations

In order to support parent-child joins, all of the (string) parent IDs
must be resident in memory (in the <<index-modules-fielddata,field data cache>>.
Additionally, every child document is mapped to its parent using a long
value (approximately). It is advisable to keep the string parent ID short
in order to reduce memory usage.

You can check how much memory is being used by the `_parent` field in the fielddata cache
using the <<indices-stats,indices stats>> or <<cluster-nodes-stats,nodes stats>>
APIS, eg:

[source,js]
--------------------------------------------------
curl -XGET "http://localhost:9200/_stats/fielddata?pretty&human&fielddata_fields=_parent"
--------------------------------------------------