class Subscription
  include Mongoid::Document
  include Mongoid::Timestamps
  include Mongoid::Paranoia

  has_many :invoices

  # worked with mongoid 2.4.3
  # no longer works with mongoid 3.0.19
  def clear_invoices!
    invoices.recurring.pending.delete_all
  end

  # works with mongoid 3.0.19
  def clear_invoices_new!
    invoices.delete_all(type: 'recurring', status: 'pending')
  end
end

class Invoice
  include Mongoid::Document
  include Mongoid::Timestamps
  include Mongoid::Paranoia

  field :type, type: String, default: 'recurring'
  field :status, type: String, default: 'pending'

  belongs_to :subscription

  scope :recurring, where(type: 'recurring').order_by(:created_at.desc)
  scope :pending, where(status: 'pending').order_by(:created_at.desc)
end

 ruby
band.members.
  where(name: "Fletch").delete_all

irb(main):032:0> sub.invoices.count
  MOPED: 127.0.0.1:27017 COMMAND      database=mongoid command={:count=>"invoices", :query=>{"deleted_at"=>nil, "subscription_id"=>"51003ff7ab0a5d3f10000001"}} (0.7582ms)
=> 1


irb(main):033:0> sub.clear_invoices!
  MOPED: 127.0.0.1:27017 COMMAND      database=mongoid command={:count=>"invoices", :query=>{"deleted_at"=>nil, "subscription_id"=>"51003ff7ab0a5d3f10000001", "type"=>"recurring", "status"=>"pending"}} (0.7679ms)
  MOPED: 127.0.0.1:27017 DELETE       database=mongoid collection=invoices selector={"$query"=>{"deleted_at"=>nil, "subscription_id"=>"51003ff7ab0a5d3f10000001", "type"=>"recurring", "status"=>"pending"}, "$orderby"=>{"created_at"=>-1}} flags=[] (0.2458ms)
=> 1
irb(main):034:0> sub.invoices.count
  MOPED: 127.0.0.1:27017 COMMAND      database=mongoid command={:count=>"invoices", :query=>{"deleted_at"=>nil, "subscription_id"=>"51003ff7ab0a5d3f10000001"}} (0.8352ms)
=> 1


irb(main):035:0> sub.clear_invoices_new!
  MOPED: 127.0.0.1:27017 COMMAND      database=mongoid command={:count=>"invoices", :query=>{:type=>"recurring", :status=>"pending", "deleted_at"=>nil, "subscription_id"=>"51003ff7ab0a5d3f10000001"}} (1.0450ms)
  MOPED: 127.0.0.1:27017 DELETE       database=mongoid collection=invoices selector={:type=>"recurring", :status=>"pending", "deleted_at"=>nil, "subscription_id"=>"51003ff7ab0a5d3f10000001"} flags=[] (0.1462ms)
  MOPED: 127.0.0.1:27017 QUERY        database=mongoid collection=invoices selector={"deleted_at"=>nil, "subscription_id"=>"51003ff7ab0a5d3f10000001"} flags=[:slave_ok] limit=0 skip=0 batch_size=nil fields=nil (0.3572ms)
=> 1
irb(main):036:0> sub.invoices.count
  MOPED: 127.0.0.1:27017 COMMAND      database=mongoid command={:count=>"invoices", :query=>{"deleted_at"=>nil, "subscription_id"=>"51003ff7ab0a5d3f10000001"}} (0.8490ms)
=> 0