has_one through does not provide `create_child` method?

gistfile1.rb

class User < ActiveRecord::Base
  has_one :image_assignment, as: :imageable
  has_one :image, through: :image_assignment
end

class ImageAssignment < ActiveRecord::Base
  belongs_to :image
  belongs_to :imageable, polymorphic: true
end

class Image < ActiveRecord::Base
  mount_uploader :image, ImageUploader
end

u = User.new
u.create_image #=> undefined method `create_image'