How do I create a rails migration to remove/change precision and scale on decimal?

format :

change_column(table_name, column_name, type, options): Changes the column to a different type using the same parameters as add_column.

First in you terminal:

rails g migration change_numeric_field_in_my_table

Then in your migration file:

class ChangeNumbericFieldInMyTable < ActiveRecord::Migration
  def self.up
   change_column :my_table, :revenue_per_transaction, :decimal, :precision => give whatever, :scale => give whatever
  end
end

then

run rake db:migrate

Source : http://api.rubyonrails.org/classes/ActiveRecord/Migration.html