Where a mapping comes from — the behaviour behind the :mapping option.

Casting needs the mapping of the index a document belongs to. By default a backend package asks the cluster, through its mapping cacher (Dowser.OpenSearch.MappingCacher, Dowser.Elasticsearch.MappingCacher), which implements this behaviour. :mapping names another source:

• a map — the mapping itself, whatever the index;
• a module implementing this behaviour — mappings read from files, from another cache, built at runtime…;
• a document module (see Dowser.Schema) — its own mapping, with no request to the cluster.

Implementing it

defmodule MyApp.Mappings do
  @behaviour Dowser.Mapping

  @impl true
  def fetch_mapping("posts" <> _rest, _repo_config), do: {:ok, MyApp.Mappings.Posts.mapping()}
  def fetch_mapping(_index, _repo_config), do: {:ok, nil}
end

index is the index a document came from, or the one a request writes to — nil when there is none. repo_config is the request's repo, resolved; use it to tell clusters apart.

Return {:ok, mapping}, {:ok, nil} when there is no mapping to be had (the document is then left as it is), or {:error, reason} when the mapping exists but couldn't be fetched — which :mapping_failure then decides about.