Dynamic attributes

.dynamic_attributes_explanation

If you have a method or attribute in a class, you could invoke the send() method in order to call those 
methods or attributes.
 
In my case, I was confused when I was trying to do a validator for my class Transaction which has 
another class associated Account, and there is a class called Entity.
 
I needed validate that an specific account was created in the Account model and belonged to the
same entity.
 
====
Contribution by [@rderoldan1](https://gist.github.com/rderoldan1)

account.rb

class Account < ActiveRecord::Base
 
  self.table_name = "accounts"
  attr_accessible :account, :entity_id
  
end

transaction.rb

class Transaction < ActiveRecord::Base
  
  # This is the method which goes to the Account model and verifies the entity and the account(attr)
  def validation_account_entity(attr)
    errors.add(attr, " not configured") unless Account.exists?(:entity_id => self.entity_id, :account => self.send(attr))
  end
  
  self.table_name = "transactions"
  
  attr_accessible :account_id, :entity_id
  
  validates :account_id, :presence => true
  
  # send the attribute to validate, you can't forget the colon before it
  validate "validation_account_entity(:account_id)"
  
end