40_Parent_child.asciidoc

Parent-Child Relationship

The parent-child relationship is similar in nature to the nested model: both allow you to associate one entity with another. 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. The advantages that parent-child has over nested objects 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.

The parent-child ID maps are stored in [docvalues], which allows them to execute quickly when fully hot in memory, but scalable enough to spill to disk when the map is very large.

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. 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 application-side-joins or data denormalization 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:

PUT /company
{
  "mappings": {
    "branch": {},
    "employee": {
      "_parent": {
        "type": "branch" (1)
      }
    }
  }
}

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