class ApiUser < ActiveRecord::Base
  has_many :posts
  validates :secret_key, presence: true
  validates :secret_algorithm, presence: true
end

class Post < ActiveRecord::Base
  API_FIELDS = [:title, :body].sort  # keep them in alphabetic order!
  attr_accessible :api_user_id, :hmac
  attr_accessible *MESSAGE_FIELDS
  belongs_to :api_user

  # these have same meaning (supports Lambdas and Symbols evaluating):
  validate :hmac, precence: true, hmac: {
    key:        lambda { api_user.secret_key },
    data:       lambda { API_FIELDS.collect{|m| send(m) }.join },
    algorithm:  lambda { api_user.secret_algorithm }
  }

  validates :hmac, presence: true, hmac: {
    key:        :'api_user.secret_key',
    data:       API_FIELDS,
    algorithm:  :'api_user.secret_algorithm'
  }

  # these are not evaluated (presumed that static value is written)
  validates :hmac, presence: true, hmac: {
    key:        'all_the_time_same',
    data:       'why you would like to have a static value here?',
    algorithm:  'md5' # by default its sha1
  }
end