Rails migration rollback for bad deploy

db-safety.sh

#!/bin/bash

# Run this script before deploying to make a text file containing all 
# schema_migrations.
#
# After deploying, if a rollback is required, run this script again 
# with an argument of 'back' to cherry-pick new migrations out of 
# schema migrations (ones that weren't in the pre-deploy log of 
# schema_migrations) and reverse them with rake db:migrate:down VERSION=X.

PRE_DEPLOY_MIGRATIONS=schema_migrations_pre.txt
POST_DEPLOY_MIGRATIONS=schema_migrations_post.txt
DB_PW=my_pw
DB_USER=my_user
DB_HOST=my_host
DB_PORT=my_port
DB_NAME=my_name

log_migrations () {
  mysql -h$DB_HOST -P$DB_PORT -u$DB_USER -p -e 'select * from $DB_NAME.schema_migrations' > $1
}

case "$1" in
# Rollback migrations
post)
  echo ROLLING BACK MIGRATIONS AFTER BAD DEPLOY
  log_migrations $POST_DEPLOY_MIGRATIONS
  declare -a migs
  migs=diff $PRE_DEPLOY_MIGRATIONS $POST_DEPLOY_MIGRATIONS | grep \> | cut -f2 -d' '
  migs_ct=`echo $migs | wc -w`
  echo $migs_ct migrations to roll back
  for mig in $migs; do
    echo Rolling back migration $mig
    RAILS_ENV=production rake db:migrate:down $mig
  done
  ;;
# Log existing migrations prior to deploy
pre)
  echo LOGGING MIGRATIONS PRIOR TO DEPLOY
  log_migrations $PRE_DEPLOY_MIGRATIONS
  ;;
# Echo usage
*)
  echo "Usage: ./this.sh <pre|post>"
esac