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

The `has_child` query works the same as the
<<query-dsl-has-child-filter,has_child>> filter,
by automatically wrapping the filter with a
<<query-dsl-constant-score-query,constant_score>>
(when using the default score type). It has the same syntax as the
<<query-dsl-has-child-filter,has_child>> filter:

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

An important difference with the `top_children` query is that this query
is always executed in two iterations whereas the `top_children` query
can be executed in one or more iteration. When using the `has_child`
query the `total_hits` is always correct.

[float]
==== Scoring capabilities

The `has_child` also has scoring support. The
supported score types are `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

coming[1.3.0]

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

With the current implementation, all `_parent` field values and all `_id`
field values of parent documents are loaded into memory (heap) via field data
in order to support fast lookups, so make sure there is enough memory for it.