A very simplistic implementation of a point ledger class that adds and deducts by creating user point transactions.

point_ledger.rb

class PointLedger
  attr_reader :student

  def initialize(student)
    @student = student
  end

  def add(amount, item)
    create_point_transaction(amount, item)
    student.points += amount
    student
  end

  def deduct(amount, item)
    create_point_transaction(-amount, item)
    student.points -= amount
    student
  end

  private
  def create_point_transaction(amount, item)
    student.point_transactions.build(amount: amount, item_type: item.class.to_s, item_name: item.name)
  end
end

purchase_calc.rb

class PurchaseCalc
  def possible?(points, item_price)
    points >= item_price
  end
end