Conditional Callback in Rails' model?

class LineItem < ActiveRecord::Base
  before_create :set_invoice_id, unless: :invoice_id

  def set_invoice_id
    self.invoice_id = line_item_group.invoice_id
  end
end

Yes you can.

class Order < ActiveRecord::Base
  before_save :normalize_card_number, :if => :paid_with_card?
end

See http://guides.rubyonrails.org/active_record_callbacks.html#conditional-callbacks for more on this topic.