gistfile1.txt

class Expense
  
  include MongoMapper::Document
  key :expense_id. ObjectId
  key :amount, Integer
  key :date, Date
  key :notes, String
  key :tags, Array
  key :category, Array

end

MongoMapper.database = 'expense'

Expense.collection.remove

Expense.create(:amount => '25.00', 
               :date => Time.parse('08/26/2010'),
               :notes => 'An Expense', 
               :tags => %w[Company1 Company 2],
               :category => %w[Freight Web Hosting])

I would avoid using Date. Just use Time. Date's are not native to mongo but times are.