After removing some Rails migrations today I couldn’t rollback my database and I realised I had no idea how Rails keeps track of which migrations it has run.

I found out they live in a Rails managed table schema_migrations, in a single VARCHAR column version (containing the datetime of each migration as a 16 digit string).

Knowing that isn’t particularly helpful because if you manually delete entries from that table you also need to manually revert the changes made in the corresponding migration. I do like to understand the technology behind the magic though.

If you find yourself in a similar situation what you probably want, and what I ended up doing after chasing this little tangent, is rails db:migrate:reset.