Nice tips about rails

Add a column in a migration and populate it immediately after.
reset_column_information

Example

    class AddNameToUser < ActiveRecord::Migration
        def self.up
            add_column :users, :name, :string
            User.reset_column_information
            User.find(:all).each do |user|
                user.name = user.first_name + ' ' + user.last_name
            end
            remove_column :users, :first_name
            remove_column :users, :last_name
        end
    end

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