Migration Tools

Rake tasks for grouping migrations.

The migration tools allow you to specify a group in your migrations. This is used to allow you to run your migrations in groups, as opposed to all at once. This is useful if you want to run a certain group of migrations before a deploy, another group during deploy and a third group after deploy.

We use this technique to be able to QA new production code in an isolated environment that runs against the production database. It also reduces the number of moving parts come deploy time, which is helpful when you're doing zero downtime deploys.

You specify which group a migration belongs to inside the migration, like so:

  class CreateHello < ActiveRecord::Migration
    group :before

    def self.up
      ...
    end
  end

The names of the possible groups are predefined to avoid turning this solution in to a generic hammer from hell. You can use the following groups: before, during, after, change. We define these as:

before this is for migrations that are safe to run before a deploy of new code, e.g. adding columns/tables

during this is for migrations that require the data structure and code to deploy "synchronously"

after this is for migrations that should run after the new code has been pushed and is running

change this is a special group that you run whenever you want to change DB data which you'd otherwise do in script/console

The list commands

  $ rake db:migrate:list - shows pending migrations by group
  $ rake db:migrate:list:before - shows pending migrations for the before group
  $ rake db:migrate:list:during - shows pending migrations for the during group
  $ rake db:migrate:list:after  - shows pending migrations for the after group
  $ rake db:migrate:list:change - shows pending migrations for the change group

The group commands

  $ GROUP=before rake db:migrate:group - runs the migrations in the specified group
  $ rake db:migrate:group:before - runs pending migrations for the before group
  $ rake db:migrate:group:during - runs pending migrations for the during group
  $ rake db:migrate:group:after  - runs pending migrations for the after group
  $ rake db:migrate:group:change - runs pending migrations for the change group

Note that rake db:migrate is entirely unaffected by this.

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