I need to make changes to an in-use production database. Just adding a few columns. I've made the changes to the dev database with migrations. What is the best way to update the production database while preserving the existing data and not disrupting operation too much?
It's MYSQL and I will be needing to add data to the columns as well for already existing records. One column can have a default value (it's boolean) but the other is a timestamp and should have an arbitrary backdated value. The row counts are not huge.
So if I use migrations how do I add data and how do I get it to just do the two (or three - I add data -latest migrations on the production db when it wasn't initially built via migrations (I believe they used the schema instead)?
It sounds like you're in a state where the production db schema doesn't exactly match what you're using in dev (although it's not totally clear). I would draw a line in the sand, and get that prod db in a better state. Essentially what you want to do is make sure that the prod db has a "schema_info" table that lists any migrations that you >don't< ever want to run in production. Then you can add migrations to your hearts content and they'll work against the production db.
Once you've done that you can write migrations that add schema changes or add data, but one thing you need to be really careful about is that if you add data using a migration, you must define the model within the migration itself, like this:
class AddSomeColumnsToUserTable < ActiveRecord::Migration
class User < ActiveRecord::Base; end
def self.up
add_column :users, :super_cool, :boolean, :default => :false
u = User.find_by_login('cameron')
u.super_cool = true
u.save
end
def self.down
remove_column :users, :super_cool
end
end
The reason for this is that in the future, you might remove the model altogether, during some refactoring or other. If you don't define the user class on line "User.find_by_login..." the migration will throw an exception which is a big pain.