I feel evil. [Guess namespaced associations automatically.]

association_class_name_guesser.rb

class ActiveRecord::Reflection::AssociationReflection
  class AmbiguousAssociationClassError < Exception
    def initialize results
      super "Could not guess association, please specify using class_name. Likely options: #{results.join(', ')}"
    end
  end

  def klass
    super
  rescue NameError
    @klass ||= ActiveRecord::Base.descendants.select do |model|
      model.name.demodulize.parameterize('_') == class_name.parameterize('_')
    end.tap do |results|
      raise AmbiguousAssociationClassError.new(results) if results.many?
    end.first
  end
end

project.rb

class Project < ActiveRecord::Base
  class Attachment < ActiveRecord::Base
    # Yes, we're smart enough to guess the Project::Discussion class_name
    belongs_to :discussion  
  end
  
  class Discussion < ActiveRecord::Base
    belongs_to :project
    
    has_many :attachments  
  end
  
  belongs_to :user
  
  has_many :discussions
  
  has_many :attachments,
    through: :discussions
    
end
  

user.rb

class User < ActiveRecord::Base
  has_many :projects
  
  has_many :discussions,
    through: :projects
    
end