comments_controller.rb

class CommentsController < ApplicationController
  def users_comments
    # SOLUTION 1: a bit complicated to add pagination and its sql performance is bad when there is a lot of comments, 
    #   because the sql query is returning all matched comments
    @user_comments = Post.all.includes(:comments).eager_load(comments: :author)
                   .where(author: {username: params[:username]}).map(&:comments)
                   .flatten

    # SOLUTION 2: easy to add pagination and its sql performance is much better 
    @user_comments = Comment.joins(:post, :author).where(author: {username: params[:username]})
  end
end