问题
I wrote a few migrations for my Rails 3 app, but I would like to change the order of the migrations. How can I change the migration order or sequence? Is it as simple as renaming the migration file with what appears to be the timestamp?
I know this is an odd question, but basically, I made a mess of my migrations and removed some old migrations and now I need to drop a table before creating a new one. I also know I can include the drop statement in the create-the-new-table migration, but I'm curious to know how to reorder migrations.
回答1:
Yes, it runs the migrations which have not been run in the order of the prefix. In earlier versions of rails, maybe 2.1 or 2.2, they used to be numbered starting with 01, but they switched to timestamps.
There is a table which keeps track of which migrations have run. The intentions is, multiple developers my have added migrations, and checked them in version control later. So, there may be a migration which has not run, but is numbered before the highest numbered migration which has run.
If you change the migration sequence, (and I have) it's better to first down version before all the migrations you are re-sequencing. Use the VERSION option with db:migrate. If the highest numbered migration that you want to keep (not run the down) is 20120318143249, call it this way.
rake db:migrate VERSION=20120318143249
I often run the down on a migration, and re-run it until I get the details of the migration to my satisfaction. Sometimes I re-order them, when I want to work on one of them, and I want it to be the last.
回答2:
Yes, the prefix on the filename is what determines the order of execution. However, keep in mind this will only change your current system if you wipe your DB/start over.
来源：https://stackoverflow.com/questions/10456761/rails-migration-change-sequence-or-order