bug-update-set.rb

require 'mongo'
col = Mongo::Connection.new.db("bug-update-set").collection('test')
col.insert({
  :name => 'hello',
  :data_stats => [
    {:name => 'first', :_type => 'big'},
    {:name => 'second', :_type => 'medium'},
    {:name => 'third', :_type => 'big'},
    {:name => 'four', :_type => 'big'},
  ]
})

col.insert({
  :name => 'hello',
  :data_stats => [
    {:name => 'first', :_type => 'big'},
    {:name => 'second', :_type => 'medium'},
    {:name => 'third', :_type => 'big'},
    {:name => 'four', :_type => 'big'},
]
})

col.find({"data_stats._type" => 'big'}).each { |doc| puts doc.inspect }

col.update({"data_stats._type" => 'big'}, {"$set" => {"data_stats.$._type" => 'small'}}, {:multi => true})

p 'after update'

p col.find({"data_stats._type" => 'big'}).count()
col.find({"data_stats._type" => 'big'}).each { |doc| puts doc.inspect }