Rails migrations: String to Text and back again. It's kind of complicated to update a database column that is currently a "string" and convert it into "text." Well, it's not hard to update the column, but it can be dangerous if you need to rollback the migration - Postgresql and other databases don't like adding a limit to the column, and the `r…

20130822132243_change_product_long_description.rb

class ChangeProductLongDescription < ActiveRecord::Migration
  def up
    # simple and straightforward
    change_column :products, :long_description, :text
  end

  # but why cant it just be:
  #   change_column :product, :long_description, :string
  # ???
  # because effin databases don't like you, that's why.
  def down
  
    # create a temporary column to hold the truncated values
    # we'll rename this later to the original column name
    add_column :products, :temp_description, :string
    
    # use #find_each to load only part of the table into memory
    Product.find_each do |product|
      temp_description = product.long_description
      
      # test if the new value will fit into the truncated field
      if product.long_description.length > 255
        temp_description = product.long_description[0,254]
      end
      
      # use #update_column because it skips validations AND callbacks
      product.update_column(:temp_description, temp_description)
    end
    
    # delete the old column since we have all the data safe in the
    # temp_description
    remove_column :products, :long_description
  
    # rename the temp_column to our original column name
    rename_column :products, :temp_description, :long_description
  end
end