Task to checkout a branch in rails and apply or rollback the migrations between the two.

git-checkout.rake

# encoding: UTF-8
#
#     $ rake checkout new_branch_name
#
# Via https://gist.github.com/jonlemmon/4076864
#
# 1. Roll back any migrations on your current branch which do not exist on the
#    other branch
# 2. Discard any changes to the db/schema.rb file
# 3. Check out the other branch
# 4. Run any new migrations existing in the other branch
# 5. Update your test database

desc "Switch git branches and apply migrations"
task "checkout" do

  # We can't checkout if we're dirty
  unless `git diff --shortstat`.blank?
    $stderr.puts "error: Your local changes would be overwritten by checkout."
    $stderr.puts "Please, commit your changes or stash them before you can switch branches."
    abort "Aborting"
  end

  # Last argument trick
  # Via http://itshouldbeuseful.wordpress.com/2011/11/07/passing-parameters-to-a-rake-task/
  task ARGV.last do ; end
  branch_name = ARGV.last

  # List migrations
  changes = `git diff #{branch_name} --name-status`.lines
  migrations = changes.map { |change|
    match = /^A.*migrate\/([0-9]+)/.match(change)
    match ? match[1] : nil
  }.compact

  # Drop it like it's hot
  begin
    should_reconnect = ActiveRecord::Base.connection_pool.active_connection?
    migrations.sort.reverse.each do |migration|
      ActiveRecord::Migrator.down(ActiveRecord::Migrator.migrations_path, migration.to_i)
    end
    
    # Switch
    sh "git checkout db/schema.rb"
    sh "git checkout #{branch_name}"

    ActiveRecord::Migrator.migrate(ActiveRecord::Migrator.migrations_path)
    ActiveRecord::Schema.verbose = false
    ActiveRecord::Tasks::DatabaseTasks.load_schema_for ActiveRecord::Base.configurations['test'], :ruby, Rails.root.join('db','schema.rb')
  ensure
    if should_reconnect
      ActiveRecord::Base.establish_connection(ActiveRecord::Base.configurations[ActiveRecord::Tasks::DatabaseTasks.env])
    end
  end
end