Many Skinny Methods

class Expense < ActiveRecord::Base
  belongs_to :payee
  protected

    # Nice and concise, but what happens as we add more rules
    # and how do we write test cases for the four different possible 
    # validation states?
    def validate
      errors.add("Not enough funds") if payee.balance - amount > 0
      errors.add("Charge is too great") if payee.account.maximum_allowable_charge > amount
    end
end

class Expense < ActiveRecord::Base
  belongs_to  :payee

  # Instead of one large validation method, break each individual
  # rule into methods, and declare them here.
  validate      :ensure_balance_is_sufficient_to_cover_amount
  validate      :amount_does_not_exceed_maximum_allowable_charge

  protected

    # These validation callbacks simply add error messages if a particular 
    # condition is met.   Each of them can be tested and understood on their own
    # without having to understand the entire body of the validate method.
    def ensure_balance_is_sufficient_to_cover_amount
      errors.add("Not enough funds") if insufficient_funds?
    end

    def amount_does_not_exceed_maximum_allowable_charge
      errors.add("Charge is too great") if exceeds_maximum_allowable_charge?
    end

    # By defining separate predicate methods we can test each of them individually
    # and new programmers can see the intent of the code, not just the implementation.


    # Instead of subtracting the amount from the balance and checking if the
    # value is greater than 0,  change the implementation to mirror the intent.
    # There was a bug in the before code,  this is more obvious.
    def insufficient_funds?
      amount > payee.balance
    end

    def exceeds_maximum_allowable_charge?
      payee.account.maximum_allowable_charge > amount
    end
end