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:

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

The has_child also has scoring support. The supported score modes are min, max, sum, avg or none. The default is none and yields the same behaviour as in previous versions. If the score mode 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:

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

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:

{
    "has_child" : {
        "type" : "blog_tag",
        "score_mode" : "sum",
        "min_children": 2, 
        "max_children": 10, 
        "query" : {
            "term" : {
                "tag" : "something"
            }
        }
    }
}

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

Parent documents can’t be sorted by fields in matching child documents via the regular sort options. If you need to sort parent document by field in the child documents then you can should use the function_score query and then just sort by _score.

Sorting blogs by child documents' click_count field:

{
    "query": {
        "has_child" : {
            "type" : "blog_tag",
            "score_mode" : "max",
            "query" : {
                "function_score" : {
                    "script_score": {
                        "script": "_score * doc['click_count'].value"
                    }
                }
            }
        }
    }
}