class TestModel
  include Mongoid::Document
  
  field :amount, :type => BigDecimal
  
  validates_numericality_of :amount, :allow_blank => false
end

ree-1.8.7-2011.03 :009 > a = TestModel.new
 => #<TestModel _id: 4f95a22fe628f831c9000003, amount: nil, _type: nil> 
ree-1.8.7-2011.03 :010 > a.amount = "asdf"
 => "asdf" 
ree-1.8.7-2011.03 :011 > a.valid?
 => true 
ree-1.8.7-2011.03 :012 > a.errors
 => #<OrderedHash {}>

ree-1.8.7-2011.03 :015 > a = TestModel.new
 => #<TestModel _id: 4f95a277e628f831c9000005, amount: nil, _type: nil> 
ree-1.8.7-2011.03 :016 > a.amount = "Asdf"
 => "Asdf" 
ree-1.8.7-2011.03 :017 > a.valid?
 => false 
ree-1.8.7-2011.03 :018 > a.errors
 => #<OrderedHash {:amount=>["is not a number"]}>