50_Has_child.asciidoc

Finding Parents by Their Children

The has_child query and filter can be used to find parent documents based on the contents of their children. For instance, we could find all branches that have employees born after 1980 with a query like this:

GET /company/branch/_search
{
  "query": {
    "has_child": {
      "type": "employee",
      "query": {
        "range": {
          "dob": {
            "gte": "1980-01-01"
          }
        }
      }
    }
  }
}

Like the nested query, the has_child query could match several child documents, each with a different relevance score. How these scores are reduced to a single score for the parent document depends on the score_mode parameter. The default setting is none, which ignores the child scores and assigns a score of 1.0 to the parents, but it also accepts avg, min, max, and sum.

The following query will return both london and liverpool, but london will get a better score because Alice Smith is a better match than Barry Smith:

GET /company/branch/_search
{
  "query": {
    "has_child": {
      "type":       "employee",
      "score_mode": "max",
      "query": {
        "match": {
          "name": "Alice Smith"
        }
      }
    }
  }
}

Tip	The default score_mode of none is significantly faster than the other modes because Elasticsearch doesn’t need to calculate the score for each child document. Set it to avg, min, max, or sum only if you care about the score.

min_children and max_children

The has_child query and filter both accept the min_children and max_children parameters, which will return the parent document only if the number of matching children is within the specified range.

This query will match only branches that have at least two employees:

GET /company/branch/_search
{
  "query": {
    "has_child": {
      "type":         "employee",
      "min_children": 2, (1)
      "query": {
        "match_all": {}
      }
    }
  }
}

1. A branch must have at least two employees in order to match.

The performance of a has_child query or filter with the min_children or max_children parameters is much the same as a has_child query with scoring enabled.

has_child Filter

The has_child filter works in the same way as the has_child query, except that it doesn’t support the score_mode parameter. It can be used only in filter context—such as inside a filtered query—​and behaves like any other filter: it includes or excludes, but doesn’t score.

While the results of a has_child filter are not cached, the usual caching rules apply to the filter inside the has_child filter.