Finds the documents in a response body, at any depth, and casts each one with Dowser.Source against its own index's mapping.

A document is a map with a string "_index" and a map "_source": a bare document (a GET /{index}/_doc/{id}), each of hits.hits[] (a search), responses[].hits.hits[] (a multi-search), docs[] (a multi-get), and so on. Each one's "_index" selects its mapping, so a mixed-index result is cast correctly.

A hit's inner_hits are cast too. They carry no _index of their own, so their mapping comes from the hit they belong to: _nested.field names the path into it, and an inner hit with no _nested (a has_child or has_parent join) is cast against the index mapping itself. The rest of a hit's envelope — fields, highlight, sort — has no mapping entry to be cast against, and only has its keys run through :key_fn.

Everything around the documents is envelope too: its keys go through :key_fn, its values are left alone.

With a :struct (a Dowser.Schema module), each hit's _source is loaded into that struct by its __load__/2, which also receives the hit's metadata as :meta; inner hits — parts of a document, not documents — stay maps.

:key_fn casts the keys of maps — the envelope, map sources, inner hits — and never the fields of a struct.