How not get current_user

i'm breaking my head to get the user of this situation:

a conversation model:

has_many :conversation_participants, :dependent => :destroy
  has_many :users,
    :through => :conversation_participants
  has_many :messages, :dependent => :destroy
  has_one :display_message,
    :class_name => 'Message',
    :order => 'created_at DESC'

  def participants(options={})
    if options[:not].is_a? User
      users - [options[:not]]
    else
      users
    end
  end

and conversation_participants:

belongs_to :user
belongs_to :conversation
attr_accessible :user_id

on a conversation helper:

 def self_or_other
    @conversation.conversation_participants.find_by_user_id(:not => current_user)
  end

please, someone could clear me how to get the other user inside conversation_participants model?