Rails selecting record that has particular records through has_many association

[Bazley]

I've set up a messaging system between characters as follows. Each character `has_many :conversations` with other characters `through: :chat_groups`. Fairly standard setup I expect.

character.rb

    has_many :chat_groups, class_name: "Chat",
                           foreign_key: "character_id",
                           dependent: :destroy


    has_many :conversations, through: :chat_groups, source: :conversation


    has_many :messages, as: :messageable

conversation.rb

    has_many :chat_participants, class_name: "Chat",
                                 foreign_key: "conversation_id",
                                 dependent: :destroy


    has_many :characters, through: :chat_participants, source: :character


    has_many :messages, as: :messageable

chat.rb

    belongs_to :character
    belongs_to :conversation

message.rb

    belongs_to :messageable, polymorphic: true

When a character (`@sender`) sends a message to another character (`@recipient`), I first need to check if a conversation between ONLY the two characters already exists, and if not, to create such a conversation. A conversation involving the two characters and other characters (a group chat) may already exist, and this should be rejected.

Something like

Conversation.has(@sender).and_has(@recipient).and(has_no_more_characters)

My question is, what's the best/most efficient way to query for the existence of such a conversation?