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Elastic Docs ›Elasticsearch Guide [7.6] ›Query DSL ›Joining queries

Has child query

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Has child query

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Returns parent documents whose joined child documents match a provided query. You can create parent-child relationships between documents in the same index using a join field mapping.

Because it performs a join, the has_child is slow compared to other queries. Its performance degrades as the number of matching child documents pointing to unique parent documents increases. Each has_child query in a search can increase query time significantly.

If you care about query performance, do not use this query. If you need to use the has_child query, use it as rarely as possible.

Example request

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Index setup

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To use the has_child query, your index must include a join field mapping. For example:

PUT /my_index
{
    "mappings": {
        "properties" : {
            "my-join-field" : {
                "type" : "join",
                "relations": {
                    "parent": "child"
                }
            }
        }
    }
}

Example query

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GET /_search
{
    "query": {
        "has_child" : {
            "type" : "child",
            "query" : {
                "match_all" : {}
            },
            "max_children": 10,
            "min_children": 2,
            "score_mode" : "min"
        }
    }
}

Top-level parameters for has_child

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type
(Required, string) Name of the child relationship mapped for the join field.
query
(Required, query object) Query you wish to run on child documents of the type field. If a child document matches the search, the query returns the parent document.
ignore_unmapped

(Optional, boolean) Indicates whether to ignore an unmapped type and not return any documents instead of an error. Defaults to false.

If false, Elasticsearch returns an error if the type is unmapped.

You can use this parameter to query multiple indices that may not contain the type.

max_children
(Optional, integer) Maximum number of child documents that match the query allowed for a returned parent document. If the parent document exceeds this limit, it is excluded from the search results.
min_children
(Optional, integer) Minimum number of child documents that match the query required to match the query for a returned parent document. If the parent document does not meet this limit, it is excluded from the search results.
score_mode

(Optional, string) Indicates how scores for matching child documents affect the root parent document’s relevance score. Valid values are:

none (Default)
Do not use the relevance scores of matching child documents. The query assigns parent documents a score of 0.
avg
Use the mean relevance score of all matching child documents.
max
Uses the highest relevance score of all matching child documents.
min
Uses the lowest relevance score of all matching child documents.
sum
Add together the relevance scores of all matching child documents.

Notes

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Sorting

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You cannot sort the results of a has_child query using standard sort options.

If you need to sort returned documents by a field in their child documents, use a function_score query and sort by _score. For example, the following query sorts returned documents by the click_count field of their child documents.

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