I just came across a serious bug/gotcha when using mysql.
Integer, default 0 -> set to NULL –> NULL and not 0

Remember
always set NOT_NULL and DEFAULT!

Example

change_column :order_items, :shipping, :boolean, :default=>false

#shipping is nil or false or true
OrderItem.find(:all,:conditions=>{:shipping=>false}) => [] 

change_column :order_items, :shipping, :boolean, :default=>false, :null=>false

OrderItem.find(:all,:conditions=>{:shipping=>false}) => [all]