Running parallel:migrate when using structure.sql instead of schema.rb causes invalid SQL

[alecvn]

We're experiencing an issue where, when there are migrations to run, running parallel:migrate causes our structure.sql file to contain invalid SQL.

After glancing at the code it seems like parallel_tests is just running db:migrate in parallel for each of the databases, and so the issue isn't caused by parallel_tests itself but rather by Rails.

I would just like to confirm that this understanding is correct, as it would entail that one basically can't use parallel:migrate if you're using structure.sql over schema.rb.

[grosser]

There could be an issue where the parallel migrations all write to the same file somehow.
You should be able to see the steps with rake <task> --trace.
The code looks like it's just running migrate X times, so a fix might be either:

• run migrate, run a single "db:schema:dump" or "db:structure:dump"
• save the original schema/structure, run migrate, restore it (that might be the easiest and fastest way)
  ... can you try these and see if they work ?

[neo87cs]

Rails 7 + Ruby 3.1.2
@grosser i solved following your suggestion and wrote a little script:

rm -f db/structure.sql
RAILS_ENV=test rails parallel:drop parallel:create
RAILS_ENV=test rails parallel:migrate
RAILS_ENV=test rails db:schema:dump
RAILS_ENV=test rails parallel:spec

Should be possible to automatize it by doing a wrapping rake to run on a rake task too

[grosser]

running a full migration before each test will be slow and brittle, so that's not a great approach
I'd prefer something like copying the schema, running parallel:migrate and then restoring the schema
(or finding a way to make the db tasks not dump the schema)