wrap transactions in a DDL transaction (if your database supports it)

migrate.rake

class << ActiveRecord::Base
  def ddl_transaction(&block)
    if connection.supports_ddl_transactions?
      transaction { block.call }
    else
      block.call
    end
  end
end

# delete original migrate task.
# note: this is ridiculous
Rake.application.instance_variable_get(:@tasks).delete("db:migrate")

namespace :db do
  desc "Migrate the database (options: VERSION=x, VERBOSE=false). wrap all migrations being run in a single ddl transaction so that they either all succeed or you're left back where you started"
  task :migrate => :environment do
    Rake::Task['db:migrate'].clear # clear the default task

    ActiveRecord::Base.ddl_transaction do
      ActiveRecord::Migration.verbose = ENV["VERBOSE"] ? ENV["VERBOSE"] == "true" : true
      ActiveRecord::Migrator.migrate("db/migrate/", ENV["VERSION"] ? ENV["VERSION"].to_i : nil)
    end
    Rake::Task["db:schema:dump"].invoke if ActiveRecord::Base.schema_format == :ruby
  end
end