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:

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

Note that the has_child is a slow query compared to other queries in the query dsl due to the fact that it performs a join. The performance degrades as the number of matching child documents pointing to unique parent documents increases. If you care about query performance you should not use this query. However if you do happen to use this query then use it as little as possible. Each has_child query that gets added to a search request can increase query time significantly.

Scoring capabilities

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:

GET /_search
{
    "query": {
        "has_child" : {
            "type" : "blog_tag",
            "score_mode" : "min",
            "query" : {
                "term" : {
                    "tag" : "something"
                }
            }
        }
    }
}

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:

GET /_search
{
    "query": {
        "has_child" : {
            "type" : "blog_tag",
            "score_mode" : "min",
            "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.

Ignore Unmapped

When set to true the ignore_unmapped option will ignore an unmapped type and will not match any documents for this query. This can be useful when querying multiple indexes which might have different mappings. When set to false (the default value) the query will throw an exception if the type is not mapped.

Sorting

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 should use the function_score query and then just sort by _score.

Sorting blogs by child documents' click_count field:

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