InvoiceItem model from one of my projects

InvoiceItem.rb

class InvoiceItem < ApplicationRecord
  # Constants

  # Includes
  # include AccountScope
  include AASM

  aasm column: "status", requires_lock: true do
    state :pending, initial: true
    state :closed

    event :close do
      transitions from: :pending, to: :closed
    end
  end
  # Serialization

  # Associations
  belongs_to :billed_item, polymorphic: true, optional: true
  belongs_to :invoice

  # Delegations

  # Callbacks

  # Validations
  validates :name, presence: true
  validates :unit_price, presence: true,
                    numericality: {
                      greater_than_or_equal_to: 0.000001,
                      less_than_or_equal_to: 999999.999999
                    }
  validates_numericality_of :units, only_integer: true, allow_nil: true
  validates :units, presence: true, allow_nil: true
  validates :total, presence: true,
                    numericality: {
                      greater_than_or_equal_to: 0,
                      less_than_or_equal_to: 999999.9999
                    }, allow_nil: true
  validates :started_at, presence: true, datetime: true
  validates :ended_at, datetime: true
  validates :status, length: { minimum: 1, maximum: 64 }, allow_blank: false

  # Named Scopes
  scope :pending_state, ->() { where(status: "pending") }
  scope :for_item, ->(billed_item) { where(billed_item: billed_item) }

  # Class Methods

  # Instance Methods

  def calculated_units
    hours = units || (((ended_at || Time.zone.now) - self.started_at) / 3600).to_f.ceil
    [730, hours].min
  end

  def calculated_total
    total || (calculated_units * unit_price).round(2)
  end
end