comments_controoler.rb

class CommentsController < ApplicationController
  # I assumed the usual naming for models and foreign columns 
  def users_comments
    # I don't like where( post_id: Post.all ) but if table comments 
    # contains deleted posts comments or even NULLs than we need to keep it this way 
    @user_comments = Comment.where( post_id: Post.all )
                            .where(author_id: Author.where( username: params[:username]) )

  end
end