The original developer of a Rails project I inherited decided to seed the test db with db/seed.rb and tie every test to the content of the seed. There were a lot of tests so rewriting then was not an option, not immediately.

I wrote the new tests in the canonical way but I still had the problem of all those seconds spent seeding the db even when I run a single test. I wrote a couple of scripts that dumped the db at the end of the tests (if a dump did not exist yet) and reloaded it at the beginning of the tests. This is much faster. I still have to clear the test db and reseed it when I switch branch, because I don't have a dump list branch.

Maybe I can create a template with the data in it. Or finally rewrite every single old test.

As long as you're using Postgres, I would definitely try out an approach based on template databases in your situation. It should check all the boxes that a run with `db/seed.rb` would, except an order of magnitude or two compared to what Rails would be doing (i.e. parsing a big Ruby file and loading schema in table by table).

One of the best things about LLMs is they make these sorts of refactors entirely plausible even if you're not a subject area expert. You could probably prototype this and have a patch ready in an hour or two.