How to change a scope of the accessor in model?

Hello all.
I have a table "Account" with the "value" column.
How i can change a scope for it, so that is will be only accessable in a
Account class? I've tried to overload a method, but no luck:

class Account < ActiveRecord::Base
  private
  def value=(amount)
    write_attribute(:value, amount)
    end
  end

=> nil

ll = Account.new

=> #<Account:0xb6ef598c @new_record=true,
@attributes={"name"=>nil, "entity_id"=>nil, "updated_at"=>nil, "currency_id"=>nil, "account_group_id"=>nil, "value"=>#<BigDecimal:b6eafb6c,'0.0',4
(12)>, "base_value"=>#<BigDecimal:b6eafa18,'0.0',4
(12)>, "account_num"=>nil, "created_at"=>nil}>

ll.private_methods.include? "value="

=> true

ll.value = 10

=> 10

ll.value

=> #<BigDecimal:b718d064,'0.1E2',4(8)>