[[parent-child]] == Parent-Child Relationship

The parent-child relationship is ((("relationships", "parent-child")))((("parent-child relationship")))similar in nature to the <>: both allow you to associate one entity with another. ((("nested objects", "parent-child relationships versus")))The difference is that, with nested objects, all entities live within the same document while, with parent-child, the parent and children are completely separate documents.

The parent-child functionality allows you to associate one document type with another, in a one-to-many relationship--one parent to many children.((("one-to-many relationships"))) The advantages that parent-child has over <> are as follows:

• The parent document can be updated without reindexing the children.

• Child documents can be added, changed, or deleted without affecting either the parent or other children. This is especially useful when child documents are large in number and need to be added or changed frequently.

• Child documents can be returned as the results of a search request.

Elasticsearch maintains a map of which parents are associated with which children. It is thanks to this map that query-time joins are fast, but it does place a limitation on the parent-child relationship: the parent document and all of its children must live on the same shard.

[NOTE]

At the time of going to press, the parent-child ID map is held in memory as part of <>. There are plans afoot to change the default setting to use <> by default instead.

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[[parent-child-mapping]] === Parent-Child Mapping

All that is needed in order to establish the parent-child relationship is to specify which document type should be the parent of a child type.((("mapping (types)", "parent-child")))((("parent-child relationship", "parent-child mapping"))) This must be done at index creation time, or with the update-mapping API before the child type has been created.

As an example, let's say that we have a company that has branches in many cities. We would like to associate employees with the branch where they work. We need to be able to search for branches, individual employees, and employees who work for particular branches, so the nested model will not help. We could, of course, use <> or <> here instead, but for demonstration purposes we will use parent-child.

All that we have to do is to tell Elasticsearch that the employee type has the branch document type as its _parent, which we can do when we create the index:

[source,json]

PUT /company { "mappings": { "branch": {}, "employee": { "_parent": { "type": "branch" <1> } } }

}

<1> Documents of type employee are children of type branch.