title: Updating boolean attr with ? before it is saved
body: <p>I have a <code>Message</code> model that has a boolean <code>from_posting?</code> attribute that defaults to <code>false</code>.</p>
  
  <p>In some cases, I want to set this attr to <code>true</code> before the Message is saved.</p>
  
  <pre><code>def create
    @message = current_user.sent_messages.build(message_params)
      if params[:reply] == 'true'
        @message.from_posting? = true
      end
      if @message.save
        render json: @message, status: 200, serializer: MessageSerializer
      else
        render json: @message.errors, status: 400
      end
   end
  </code></pre>
  
  <p>But <code>@message.from_posting? = true</code> throws an <code>unexpected =</code> error. I can do <code>@message.title ='something'</code>, but why can't I use <code>attr_accessor</code> to set the boolean value when the attr ends in a <code>?</code>?</p>
  
  <p>I thought about saving the record and then updating <code>from_posting?</code> if I need to, but that just seems like an extra db write. I tried using <code>@posting.toggle(from_posting?)</code> but that throws an error on <code>from_posting?</code> as well.</p>