Scope Merging.rb

class User < ActiveRecord::Base
  has_many :tasks

  def self.with_flagged_comments
    includes(:task).merge(Task.with_flagged_comments)
  end
end

class Task < ActiveRecord::Base
  belongs_to :user
  has_many :comments

  def self.with_flagged_comments
    includes(:comment).merge(Comment.flagged)
  end
end

class Comment < ActiveRecord::Base
  belongs_to :task

  def self.flagged
    where(:flagged => true)
  end
end

# User does not have the :comments association
User.with_flagged_comments
=> User.includes(:task).includes(:comments).where("comments.flagged = true")
=> "Association missing"

# What we want
=> User.includes(:task => :comments).where("comments.flagged = true")