gistfile1.rb

class Parent < ActiveRecord::Base
  has_many :children
end

class Child < ActiveRecord::Base
  has_one :parent
  has_many :sub_children

  def self.method
    # I want this to have a conditional .where("child.parent_id" => parent_id) if I called .method via the parent model, 
    # as if it were a nested scope.
    SubChildren.joins(:child).sum(....)
  end
end