According to the [Rails
Source](https://github.com/rails/rails/blob/master/activerecord/lib/active_record/railties/databases.rake#L20)
the `ActiveRecord::Migrator.migrations_paths` are overwritten with the
value of `ActiveRecord::Tasks::DatabaseTasks.migrations_paths` every
time the config is loaded.
This caused a bug for Discourse development where if you ran:
`rake db:drop db:create db:migrate` in one line, you would not get our
post migrations, as those had a custom value for `migrations_paths`.
The fix is to use `ActiveRecord::Tasks::DatabaseTasks.migrations_paths`
to set up all our custom paths. Everything seems to work as expected.
This reduces chances of errors where consumers of strings mutate inputs
and reduces memory usage of the app.
Test suite passes now, but there may be some stuff left, so we will run
a few sites on a branch prior to merging
This moves us away from the delayed drops pattern which
was problematic on two counts. First, it uses a hardcoded "delay for"
duration which may be too short for certain deployment strategies.
Second, delayed drop doesn't ensure that it only runs after
the latest application code has been deployed. If the migration runs
and the application code fails to deploy, running the migration after
"delay for" has been met will cause the application to blow up.
The new strategy allows post deployment migrations to be skipped if the
env `SKIP_POST_DEPLOYMENT_MIGRATIONS` is provided.
```
SKIP_POST_DEPLOYMENT_MIGRATIONS=1 rake db:migrate
-> deploy app servers
SKIP_POST_DEPLOYMENT_MIGRATIONS=0 rake db:migrate
```
To aid with the generation of a post deployment migration, a generator
has been added. Simply run `rails generate post_migration`.
