Based on https://www.drupal.org/project/search_api_elasticsearch_attachments/issu... → , which is an issue for 8.x-1.x.
At the moment, only fields on entity type 'node' are supported.
This is the case because of the elseif check $entity_type == 'node' in the ElasticsearchAttachments class.
I think we can just remove that check and the logic should still work for other entities.
Needs work
Code
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
Patch in 15 does not apply cleanly to the 7.x-alpha1 release. and I'm having trouble finding a commit where it does apply. There seems to have been a good deal of changes made to ElasticsearchAttachments.php, and just based on some of the code, there could be some overlap from this issue. Maybe a maintainer can confirm if anything on this was previously committed?