I've been really happy with how my current company[0] has been doing migrations and I've seen a couple others do it but it seems like it should be more widespread.
Database Schema as Code
Instead of writing up and down migrations, you define what the end state should look like. Then the computer will figure out how to get here. This is just how the industry started managing server configurations (Puppet) and infrastructure (Terraform).
We use protocol buffers so it was pretty straight forward to have a definition of what our tables should look like. We have a script that figures out what the delta is between two states (either proto files or a db) and can calculate the schema migration SQL (e.g. CREATE TABLE, etc).
From there, we run it through a safety check. Any unsafe migration (either for data loss or performance issues e.g. DROP TABLE) requires an extra approval file.
There's no real difference between an up migration and a down migration (except that one tends to result in an unsafe migrations). It's calculable at CI time so we can give devs a chance to look at what it's going to do and approve any unsafe migrations. API compatability checks enforce that you need to deprecate before you can drop.
DML, that is data changes, are handled via standard check in a sql file and CI will run it before the code deploy and after the schema migration.
Alembic is the one other place I've seen this concept (a couple others mentioned this) so it's not new, but surprised I haven't seen it more places.
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I call this declarative schema management, since the repo declares the desired state, and the tooling knows how to reach this state. This concept is finally catching on lately, although some huge companies have already been doing it this way for quite some time. Facebook is a key example; they've managed their schema changes in a pure-SQL declarative fashion, company-wide, for nearly a decade.
I'm developing a suite of tools [1] to provide declarative schema management and "schema change by pull request" functionality, initially targeting MySQL and MariaDB. A few large companies have built pipelines using one of my tools -- including Twilio SendGrid, who wrote about their process in-depth recently [2].
[2] https://sendgrid.com/blog/schema-management-with-skeema/
I'm surprised this isn't more of a thing. It seems like the natural evolution of "[X] as code". I've always been a little turned off by migrations (though they were certainly an improvement over the previous situation, which was basically just indeterministic changes on the fly).
fwiw, other declarative tools are starting to pop up -- besides my tool Skeema, some others I've seen recently are Migra [1] and sqldef [2]. And meanwhile a bunch of enterprise tools for MS SQL Server have operated in the declarative fashion for quite a long time, although usually with GUIs instead of being git / pull-request-driven. So I think/hope it's just a matter of time before this concept becomes more widely known.