Jim Ferenczi b5d62ae747 Introduce ParentJoinFieldMapper, a field mapper that creates parent/child relation within documents of the same index (#24978)
* Introduce ParentJoinFieldMapper, a field mapper that creates parent/child relation within documents of the same index

This change adds a new field mapper named ParentJoinFieldMapper. This mapper is a replacement for the ParentFieldMapper but instead of using the types in the mapping
it uses an internal field to materialize parent/child relation within a single index.
This change also adds a fetch sub phase that automatically retrieves the join name (parent or child name) and the parent id for child documents in the response hit fields.
The compatibility with `has_parent`, `has_child` queries and `children` agg will be added in a follow up.

Relates #20257
2017-05-31 18:07:21 +02:00
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