using-association-scope.rb

# app/models/user.rb
class User < ActiveRecord::Base
  has_many :posts, dependent: :destroy
  has_many :published_posts, -> { published }, class_name: 'Post'
end

# app/models/post.rb
class Post < ActiveRecord::Base
  belongs_to :user
  scope :published, -> { where(published: true) }
end