Environment

ruby 1.9.2-p180
mongoid 2.0.0

Example

require "rubygems"
require "mongoid"
require 'rspec'

Mongoid.database = Mongo::Connection.new().db('testdb')
Mongoid::logger = Logger.new(STDOUT)

class Sky
include Mongoid::Document
include Mongoid::Paranoia
references_many :clouds, :dependent => :destroy
end

class Cloud
include Mongoid::Document
referenced_in :sky
end

sky = Sky.create!
cloud = Cloud.create!(:sky => sky)

sky.destroy

Sky.count.should == 0
Cloud.count.should == 0 # => expected: 0 got: 1 (using ==)

Conclusion

:dependent => :destroy has no effect since the remove() method is overwritten by the paranoia module and the cascade! method is therefore never called.

I'm not sure how it's supposed to work but neither destroy nor destroy! calls the cascade! method but both triggers the :destroy callbacks.