Has Child Query

The has_child query works the same as the has_child filter, by automatically wrapping the filter with a constant_score (when using the default score type). It has the same syntax as the has_child filter:

{
    "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.

The has_child also has scoring support from version 0.20.2. 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_type field inside the has_child query:

{
    "has_child" : {
        "type" : "blog_tag",
        "score_type" : "sum",
        "query" : {
            "term" : {
                "tag" : "something"
            }
        }
    }
}

The _scope support has been removed from version 0.90.beta1. See: https://github.com/elasticsearch/elasticsearch/issues/2606

A _scope can be defined on the filter allowing to run facets on the same scope name that will work against the child documents. For example:

{
    "has_child" : {
        "_scope" : "my_scope",
        "type" : "blog_tag",
        "query" : {
            "term" : {
                "tag" : "something"
            }
        }
    }
}

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.