http://www.elasticsearch.org/guide/reference/query-dsl/has-child-query.html
        The has_child query accepts a query and the child type to run against, and results in parent documents that have child docs matching the query.

        > child_query = ElasticQuery().term(tag='something')
        > query = ElasticQuery().has_Child('blog_tag', child_query)

def has_child(cls, child_type, query):
        '''
        http://www.elasticsearch.org/guide/reference/query-dsl/has-child-query.html
        The has_child query accepts a query and the child type to run against, and results in parent documents that have child docs matching the query.

        > child_query = ElasticQuery().term(tag='something')
        > query = ElasticQuery().has_Child('blog_tag', child_query)
        '''

        instance = cls(has_child={'type': child_type, 'query': query})
        return instance