Rails selecting record that has particular records through has_many association

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?