combine.rb

chat: {
  id: 1,
  owner_id: 1,
  member_id: 2
}

user1: {
  id: 1  
  name: 'Foo'
}

user2: {
 id: 2,
 name: 'Bar'
}

ChatMapper do
  combine :users, on: [:owner_id, :member_id] => :id do
    model User
  end
end

chat.users => [user1, user2]