recently in Mongoid gem there have been added possibility to include nested models: User.includes(:address, friends: [:address, :followers]) (https://jira.mongodb.org/browse/MONGOID-4173)

Lets say I have following models:

class Pages::Continent
has_many :contries, class_name: "Pages::Country"
end

class Pages::Country
belongs_to :continent, class_name: "Pages::Continent"
has_many :contries, class_name: "Pages::Cities"
end

class Pages::City
belongs_to :continent, class_name: "Pages::Country"
end
While trying to select continents including countries and cities:

@continents = Pages::Continent.includes(countries: [:cities])
I receive the error: NameError (uninitialized constant Country) which probably means that it does not look for Pages::Country but just for Country model (however as you can see class_name attributes are specified for each model)