#!/usr/bin/env ruby
require 'mongoid'
DB_NAME = 'test_delete_all'
class Band
  include Mongoid::Document
  has_many :members
end
class Member
  include Mongoid::Document
  field :name, type: String
  belongs_to :band
end
Mongoid.load_configuration(sessions: { default: { database: DB_NAME,
                                                  hosts:    ['localhost'],
                                                  options:  {} } })
b = Band.create
bid = b.id.to_s
alice = b.members.create(name: 'alice')
bob   = b.members.create(name: 'bob')

puts "Band members count: #{Member.where(band_id: bid).count}"
puts "Deleting: #{Member.delete_all(band_id: bid)}"

ruby test.rb
Band members count: 2
Deleting: 0