class Car
  include Mongoid::Document
  include Mongoid::Timestamps::Created
  field :name, type: String
  #...
  embeds_many :car_images, cascade_callbacks: true
end

class CarImage
  include Mongoid::Document
  include Mongoid::Timestamps::Created
  field :main, type: Boolean, default:false
  mount_uploader :image, CarImageUploader, mount_on: :image_filename
  validates :image, :file_size => {:maximum => 5.megabytes.to_i}, :allow_blank => false
  embedded_in :car
end

Car.find_by(name:'Susie').car_images

[
    [0] #<CarImage:0x007f821a372aa0> {
             :_id => "511458085d39f373cb00001f", :main => false
    },
    [1] #<CarImage:0x007f821a371f38> {
             :_id => "511458085d39f373cb000020", :main => false
    },
    [2] #<CarImage:0x007f821a3795f8> {
             :_id => "511458085d39f373cb000021", :main => false
    },
    [3] #<CarImage:0x007f821a378540> {
             :_id => "511458085d39f373cb000022", :main => true
    }
]

Car.find_by(name:'Susie').car_images.destroy_all
4
 => nil 

Car.find_by(name:'Susie').car_images.first.destroy
true
 => nil 

test_delete = Car.find_by(name:'Susie').car_images.first.destroy
test_delete.errors
 => #<ActiveModel::Errors:0x007f821a2f9100 @base=#<CarImage _id: 511458015d39f373cb00001e, _type: nil, created_at: nil, main: false, image_filename: nil>, @messages={}>