Rails activerecord query counting records via has_many :through

[Bazley]

A character has_many :conversations, through: :chats, and each conversation has_many :characters, through: :chats.

character.rb

    has_many :chats, foreign_key: "character_id", dependent: :destroy
    has_many :conversations, through: :chats, source: :conversation
    has_many :messages

conversation.rb

    has_many :chats, foreign_key: "conversation_id", dependent: :destroy
    has_many :characters, through: :chats, source: :character
    has_many :messages

chat.rb

    belongs_to :character
    belongs_to :conversation

messages_controller.rb

    def create
      @sender    = Character.find_by(callsign: params[:callsign])
      @recipient = Character.find_by(callsign: params[:recipient])
      # 1
      @senderConversations = @sender.conversations
      # 2
      @senderAndRecipientConversations = @senderConversations.joins(:characters).where(characters: {id: @recipient.id}) # works fine up to here
      # 3
      @conversationAssociation = @senderAndRecipientConversations.joins(:characters).group('conversations.id').having('count(characters.id) = ?', 2) # problem line!
      @conversation = @conversationAssociation.first
      if(@conversation.nil?)
        @conversation = @sender.conversations.create
        @recipient.conversations << @conversation
      end
      @message = @sender.messages.build(message_params.merge(:conversation => @conversation))
      if @message.save
        @conversation.messages << @message
        respond_to do |format|
          format.html do
          end
          format.js do
          end
        end
      else
        redirect_to request.referrer || root_url
      end
    end

All conversations must have at least two characters, and some conversations may have more than two characters (group chats).

I'm trying to construct a 3-step query that first finds all the @sender's conversations, then selects from these the ones that also involve the @recipient, and finally chooses the conversation which only involves @sender and @recipient and no other characters.

The first two steps are working fine. I can't get the code for the third step right. The best I've managed is:

    @senderAndRecipientConversations.joins(:characters).group('conversations.id').having('count(characters.id) = ?', 2)

What's wrong with this code?