Rails rake db:migrate has no effect

Question

I made a new Rails 3 app today, added a simple migration, and for some reason, nothing happens when I do rake db:migrate. It simply pauses a few seconds, then returns to the command prompt, with no errors or anything. Schema.rb and the database stay empty.

Any ideas what could be going on? I've made many apps and never had this problem. Everything is a totally standard setup too.

Answer 1

There's a few reasons why your migrations won't run, but the most common is that the system is already under the impression that all the migrations you've defined have already run.

Each migration creates an entry in the schema_migrations table with the version column corresponding to the identifier number. If you want to force a migration to re-run you can usually back it out and retry it. For example, if you had 20100421175455_create_things.rb then you would re-run it using:

rake db:migrate:redo VERSION=20100421175455

A common situation is that your migration has failed to run in the first place, that it generated an exception for instance, and yet Rails still considers it complete. To forcibly re-run a migration, delete the corresponding record from the schema_migrations table and run rake db:migrate again.

One way to avoid this kind of problem in the future is to define your migrations with an automatic back-out procedure:

class CreateThings < ActiveRecord::Migration
  def self.up
    # ... (migration) ...

  rescue
    # If an exception occurs, back out of this migration, but ignore any
    # exceptions generated there. Do the best you can.
    self.down rescue nil

    # Re-raise this exception for diagnostic purposes.
    raise
  end
end

If you have a mistake in your migration you will see the exception listed on the console. Since the migration has automatically been rolled back you should be able to run it again and again until you get it right.