add_double_to_active_record.rb

module CustomColumnTypes
  def double(*args)
    if !args.last.is_a? Hash
      args.push({})
    end

    args.last[:limit] = 53

    float *args
  end
end

module ActiveRecord
    module ConnectionAdapters
        class Table
            include CustomColumnTypes
        end 
        class TableDefinition
            include CustomColumnTypes
        end 
    end 
end

create_very_precise_times.rb

class CreateVeryPreciseTimes < ActiveRecord::Migration
    def up
        create_table :very_precise_times do |t|
            t.datetime :measured_at
            t.double   :measured_at_frac
        end
    end

    def down
        drop_table :very_precise_times
    end
end