ui.modern.related-document-fetch.[relatedDocumentKey].type

This option can be used to fetch documents related (by way of URLs recorded as metadata values) to the primary search results.

The key includes a relatedDocumentKey which indicates the name under which the related document will be stored within the response.resultPacket.results.relatedDocuments section of the data model.

The value must be one of the below forms which define the location to expect URLs of related documents:

In terms of the data model, the two above cases translate to:

None

If you have a 'previousMessageUrl' field which contains the URL of the previous message in some conversation stream, the following configuration would load it under the name 'previousMessage' with each search result.

If you wished to then load the previous message to that, the following configuration would load the next previous message from the related document loaded above.

This process can be repeated for many metadata classes, and many 'hops' out from the initial search result, however be aware that such lookups can become slow if many are performed for a request.