# user.rb
# simple
has_many :posts, dependent: :destroy # all posts
has_many :published_posts, -> { where(published: true) }, class_name: "Post" # all published posts

# advanced
has_many :comments, through: :posts,  dependent: :destroy
has_many :comments_on_published_posts, through: :published_posts, dependent: :destroy # comments on published posts