class Invoice < ActiveRecord::Base
  has_many :invoice_lines
  def readonly?; self.invoice_sent?  end
end

def InvoiceLine < ActiveRecord::Base
  def readonly?; self.invoice.readonly?  end
end

def create_or_update
  raise ReadOnlyRecord if readonly?
  ...
end

original = line.readonly?
line.readonly? = lambda: false
line.save()
line.readonly? = original

line = InvoiceLine.last
def line.readonly?; false; end

line.update_columns(description: "Compliments cost nothing", amount: 0)

InvoiceLine.where(description: "Free Stuff Tuesday").update_all(amount: 0)

class InvoiceLine < ActiveRecord::Base
  attr_accessor :force_writeable

  def readonly?
    invoice.readonly? unless force_writeable
  end
end

line.force_writable = true
line.update(description: "new narrative line")

class InvoiceLine < ActiveRecord::Base
  def force_update(&block)
    saved_force_update = @_force_update
    @_force_update = true
    result = yield
    @_force_update = saved_force_update
    result
  end

  def readonly?
    invoice.readonly? unless @_force_update
  end
end

line.force_update do
  line.update(description: "new description")
end

class InvoiceLine < ActiveRecord::Base
  validate :readonly_policy

  def readonly_policy
    if invoice.readonly?
      (changed - ["description", "amount"]).each do |attr|
        errors.add(attr, "is a read-only attribute")
      end
    end
  end
end

invoice_line1.instance_variable_set(:@item_num, 123)