Change all datetime columns to have a precision of 0, to match the default precision of these columns in MySQL. This is useful if you migrate a database from MySQL to PostgreSQL and don't need the extra microsecond precision (precision: 6) that is the default in PostgreSQL (see http://www.postgresql.org/docs/9.2/static/datatype-datetime.html).

reduce_precision_of_datetime_columns.rb

# Change all datetime columns to have a precision of 0, to match the default precision of these
# columns in MySQL.
#
# This is useful if you migrate a database from MySQL to PostgreSQL and don't need the extra
# microsecond precision (precision: 6) that is the default in PostgreSQL
# (see http://www.postgresql.org/docs/9.2/static/datatype-datetime.html).
#
# Before:
#  > some_record.created_at.strftime('%Y-%m-%d %H:%M:%S.%N')
# => "2013-07-25 11:49:33.270032000"
# After:
#  > some_record.created_at.strftime('%Y-%m-%d %H:%M:%S.%N')
# => "2013-07-25 11:49:33.000000000"
#
class ReducePrecisionOfDatetimeColumns < ActiveRecord::Migration
  def up
    Rails.application.eager_load! 
    puts %(ActiveRecord::Base.descendants: #{(ActiveRecord::Base.descendants).map(&:name).inspect})

    ActiveRecord::Base.descendants.each do |model|
      model.columns.select {|_| _.type == :datetime }.map(&:name).each do |column_name|
        execute "alter table #{model.table_name} alter column #{column_name} type timestamp (0)"
      end
    end

=begin If you're on Rails 4+, you can use this instead (see https://github.com/rails/rails/pull/6821):
    ActiveRecord::Base.descendants.each do |model|
      change_table model.table_name do |t|
        model.columns.select {|_| _.type == :datetime }.map(&:name).each do |column_name|
          t.change column_name, :datetime, precision: 0
        end
      end
    end
=end

  end

  def down
  end
end