car.rb

class Car
  has_many :cross_sections, as: :collectible
end

collection.rb

class Collection
  has_many :cross_sections
  has_many :collectibles, through: :cross_sections
end

cross_section.rb

class CrossSection
  belongs_to :collectible, polymorphic: true
  belongs_to :collection
end

user.rb

class User
  has_many :collections
  has_many :favorites, -> { collections.where type: "Favorite" }
end