Default value for a float, based on related model in mongoid

I have an app, where users list can be filtered on some criteria. One such criteria is experience of the user an mentioned in professional details. I have different kind of filters at different places in app and have a model corresponding to each filter. some of them have common functionality. So I extracted the common portion in a module, which can be included in any model to get the required functionality. I am trying to follow the concept of Single Responsibility here.

# model
class Filter::WorkOpportunity
  include Mongoid::Document
  include Filter::NotificationSubFilter
  include Filter::User::Qualification
  include Filter::User::Experience

  belongs_to :college

  def matched_user_ids
    # ...
  end
end

# one of the included module I am having problems with
# others work fine
module Filter::User::Experience
  extend ActiveSupport::Concern
  DEFAULT_MAX_EXP = 10.0

  included do
    include InstanceMethods
    field :min_exp, :type => Float, :default => 0.0
    field :max_exp, :type => Float, :default => lambda { default_max_exp }
  end

  module InstanceMethods
    # Appends users criteria passed, to filter on qualifications
    def filter_experience users=nil
      # ...
    end

    def default_max_exp
      @default_max_exp ||= begin
        established_at = self.college.try(:established_at)
        if established_at.blank?
          DEFAULT_MAX_EXP
        else
          [(Time.now.year - established_at.year + 1), DEFAULT_MAX_EXP].max.to_f
        end
      end
    end
  end
end    

I get an error when I try to initialize the filter.

NameError: undefined local variable or method `default_max_exp' for Filter::WorkOpportunity:Class

Other included modules have similar structure. Even if I move the default_max_exp to the inc开发者_JS百科luded block, it still doesn't work.

While I was posting the question here I realized that it is because, the default_max_exp should be a class method, and it is an instance method. But I want the default value to instance based, as the different filters can belong to different colleges and default maximum experience should be based on college.

How can I have an instance based default?

RoR: 3.0.4, Mongoid: 2.0.2

---

I believe you can't do that. You can't respond to instance in your field class method. self for field is a Filter::WorkOpportunity class, not an instance you have created. In your case you can specify dynamic defaults for your instances in after_initialize callback or create your own getter for max_exp which will return some value which will depends on instnace state.