Rails query user first name on join model

this situation that find the user on a join model is seems tricky than usual

 class ConversationParticipant < ActiveRecord::Base
      belongs_to :user
      belongs_to :conversation
      attr_accessible :user_id
    end



   class User < ActiveRecord::Base
      has_many :conversation_participants

  end

I'm trying to find the conversation_participant with the fisrt_name with this query

 user = User.where('conversation_participant like ? or first_name like ?', query, query)

but the query did not return the user_participant and the user first_name either.

someone can spare a hint please!