Is it permissible to delete (or archive) old migration files in a Rails app if the schema is stable?
My migrations are numerous and I suspect there may be some problem in there somewhere, because I occasionally have problems migrating the database on Heroku.
You don't need to keep around your old migration files in a Rails app, because your database schema should be captured either in schema.rb or an equivalent SQL file that can be used to regenerate your schema.
Migrations are not the authoritative source for your database schema. That role falls to either db/schema.rb or an SQL file which Active Record generates by examining the database. They are not designed to be edited, they just represent the current state of the database.
There is no need (and it is error prone) to deploy a new instance of an app by replaying the entire migration history. It is much simpler and faster to just load into the database a description of the current schema, which is in schema.rb or the SQL file.
This file should be versioned and kept in source control.
To set up automatic schema.rb generation, modify config/application.rb by the config.active_record.schema_format setting, which may be :ruby or :sql.
If :ruby is selected then the schema is stored in db/schema.rb.
If :sql is selected, the schema is dumped out in native SQL format of
your database.