"Undefined method blurhash" when upgrading to 2.8.1 or later

When upgrading an instance from 2.8.0 to 2.8.1 or later, I’ve been getting a NoMethodError for MediaAttachment#blurhash:

Jun 07 04:43:04 Mastodon bundle[43027]: [6bf8af7e-8326-4f4a-bfea-fa38bc77a927] method=GET path=/api/v1/timelines/home format=html controller=Api::V1::Timelines::HomeController action=show status=500 error='NoMethodError: undefined method `blurhash' for #<MediaAttachment:0x00007f1a8ec87670>' duration=108.72 view=0.00 db=4.18

I’ve checked, and it looks like I did run the migrations properly. I do see a blurhash model on the media_attachment table. I’m having trouble figuring out what exactly might be going wrong. I have confirmed that 2.8.1, 2.8.2, and 2.8.3 all exhibit this bug with up-to-date bundles and database migrations.

This looks similar to this issue, but there unfortunately wasn’t much actionable in the followup: Issue with blurhash preventing the mastodon-web service from starting

Figured it out! Looks like db/assets.rb ended up being out of date somehow; making sure that got regenerated so the column appears fixed the problem.

I don’t understand this reply. I don’t think there is any such file as db/assets.rb?