You can send the --dry-run argument to any rake task to see what it will do.

topfunky$ rake rails:update --dry-run
  ** Invoke rails:update (first_time)
  ** Invoke rails:update:scripts (first_time)
  ** Execute (dry run) rails:update:scripts
  ** Invoke rails:update:javascripts (first_time)
  ** Execute (dry run) rails:update:javascripts
  ** Invoke rails:update:configs (first_time)
  ** Execute (dry run) rails:update:configs
  ** Execute (dry run) rails:update

Wouldn’t it be nice to have a similar feature for migrations so you can see what will happen without actually changing the database? This is experimental right now, but I’ve added a migrate_dry task to the topfunky_power_tools plugin.

It works both up and down for the built-in commands (add_column, remove_column, etc.). It will fail for other arbitrary data changes, but I hope to come up with a solution for that.

topfunky$ rake migrate_dry VERSION=6

** DRY RUN (Non-destructive) **
== AddAuthenticatedTable: reverting ===========================================
-- drop_table("users")
   -> 0.0005s
== AddAuthenticatedTable: reverted (0.0019s) ==================================

See also

At RailsConf, Glenn Vanderburg talked about how to test your migrations.

Carson Rails Workshop, Two Days, London August

My next workshop is in London and will be a two day hands-on workshop. Come learn about the basics of Rails, Test-Driven Development, plugin development, and more. → More Details

I’ll also be at the London Ruby Users Group on Aug 8. More details soon…