在项目讨论中向所有用户发送电子邮件

I'm having bit of a trouble trying to figure out how I would send an email to all Users that have made a post in a discussion Topic. I have the user_mailer setup and UserMailer.new_post(@post).deliver in the right spot in the controller. Just trying to figure out how to extract just the emails (array of) of Users that have Posts that belong to that Topic.I'm having bit of a trouble trying to figure ou