* Use ancestry in the index and add test
* Add tests for show action
* Return complete comments trees for index, kill n+1 and use correct edge cache key
* Return complete comments tree for show, kill n+1 and use correct edge cache key
* Refactor tests and add surrogate keys tests
* Preload users for the whole subtree
* Clarify explanation of the edge cache keys construction
* Refactor partials
* Remove action caching
* Replace .map.flatten with .flat_map
* Fix surrogate key prefix according to Fastly docs
* Add comment.purge to Comments::BustCacheJob to use Fastly-Rails
* Fix surrogate key spec
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