class User
  include Mongoid::Document
  field :email, type: String
  embeds_many :cars, cascade_callbacks: true
end

class Car
  include Mongoid::Document
  field :model, type: String
  field :trunk_items, type: Array, default: Array.new 
  embedded_in :user
end

Ran the following command
User.find_by(email:'test_user@test.com').cars.destroy_all
=> 1

I also ran:
test_destroy = User.find_by(email:'test_user@test.com').cars.first.destroy(validate:false)
=> true

But when I run User.find_by(email:'test_user@test.com').cars the embedded document still exists. I'm running mongoid 3.0.18