migrate roll back...

rubyonrails-talk
1

Say I table called products with 001_create_products.rb Then say I add a column called price to product using a the migration 002_add_price.rb

002_add_price.rb class AddPrice < ActiveRecord::Migration   def self.up      add_column ...   end

def self.down   remove_column ... end

Now if I decide to remove the price column, effectively, rolling BACK my migration to 001 - how does "self.down" in 002_add_price.rb get called? I'm puzzled :slight_smile:

thanks, roupen n.

2

Pretty much the same way the self.up got run going the other way.

Basically rake db:migrate uses the VERSION environment variable to determine which migration level is desired. It then looks at the schema_version in the db to determine the current version. If schema_version is less than VERSION, then it looks for migrations > schema_version and <= VERSION loads each one in ascending sequence and executes the up method in each migration.

Similarly if schema_version is > than VERSION it runs the migrations in descending sequence and executes the down method in each one.

db:rollback works basically the same way.

3

Makes sense - thanks!