Removes members from set if no corresponding key can be found (assuming "widgets" / "widget:99" naming convention)

removeExpiredElementsFromCollection.lua

local collectionKey = KEYS[1].."s"
local members = redis.call("smembers", collectionKey)
for _, id in ipairs(members) do
  if redis.call("exists", KEYS[1]..":"..id) == 0 then
    redis.call("srem", collectionKey, id)
  end
end
return redis.call("scard", collectionKey)