title: Rails - Find all entries that don't have any children with a specific value
body: <p>We're trying to create an ActiveRecord query that could be simplified to a blog example for the sake of understanding. How would we find all blog posts that don't have any comments left by a specific user?</p>
  
  <p>Our current query returns posts as long as some of the comments aren't by the specific user, but we need to exclude the blog post if any of the comments are written by that user. Any ideas?</p>