ree-1.8.7-2010.02 > coll= Mongo::Connection.new.db("test").collection("test")
=> #<Mongo::Collection:0x1067975c8 @hint=nil, @cache_time=300, @cache={}, @connection=#<Mongo::Connection:0x1067a7018 @id_lock=#<Mutex:0x1067a6b90>, @queue=#<ConditionVariable:0x1067a6af0>, @timeout=5.0, @host_to_try=["localhost", 27017], @primary_pool=#<Mongo::Pool:0x10679f840 @checked_out=[], @queue=#<ConditionVariable:0x10679f0e8>, @timeout=5.0, @size=1, @pids=

{#<TCPSocket:0x10679a6d8>=>5914}

, @connection_mutex=#<Mutex:0x10679f110>, @port=27017, @sockets=[#<TCPSocket:0x10679a6d8>], @connection=#<Mongo::Connection:0x1067a7018 ...>, @host="localhost", @socket_ops=

{#<TCPSocket:0x10679a6d8>=>[]}

>, @connection_mutex=#<Mutex:0x1067a6b68>, @op_timeout=nil, @port=nil, @logger=nil, @safe_mutexes=

{#<TCPSocket:0x10679a6d8>=>#<Mutex:0x10679a458>, #<TCPSocket:0x1067a69d8>=>#<Mutex:0x1067a0bc8>}

, @slave_ok=nil, @read_primary=true, @safe=false, @pool_size=1, @auths=[], @host=nil, @primary=["localhost", 27017]>, @db=#<Mongo::DB:0x1067981d0 @strict=nil, @cache_time=300, @connection=#<Mongo::Connection:0x1067a7018 @id_lock=#<Mutex:0x1067a6b90>, @queue=#<ConditionVariable:0x1067a6af0>, @timeout=5.0, @host_to_try=["localhost", 27017], @primary_pool=#<Mongo::Pool:0x10679f840 @checked_out=[], @queue=#<ConditionVariable:0x10679f0e8>, @timeout=5.0, @size=1, @pids=

{#<TCPSocket:0x10679a6d8>=>5914}

, @connection_mutex=#<Mutex:0x10679f110>, @port=27017, @sockets=[#<TCPSocket:0x10679a6d8>], @connection=#<Mongo::Connection:0x1067a7018 ...>, @host="localhost", @socket_ops=

{#<TCPSocket:0x10679a6d8>=>[]}

>, @connection_mutex=#<Mutex:0x1067a6b68>, @op_timeout=nil, @port=nil, @logger=nil, @safe_mutexes=

{#<TCPSocket:0x10679a6d8>=>#<Mutex:0x10679a458>, #<TCPSocket:0x1067a69d8>=>#<Mutex:0x1067a0bc8>}

, @slave_ok=nil, @read_primary=true, @safe=false, @pool_size=1, @auths=[], @host=nil, @primary=["localhost", 27017]>, @safe=false, @name="test", @pk_factory=nil>, @safe=false, @name="test", @pk_factory=BSON::ObjectId>
ree-1.8.7-2010.02 > h=

{'a' => 'b','c' => 'd'}

=>

{"a"=>"b", "c"=>"d"}

ree-1.8.7-2010.02 > coll.insert(h)
=> BSON::ObjectId('4da86d5811a03c171a000001')
ree-1.8.7-2010.02 > coll.find_one
=>

{"_id"=>BSON::ObjectId('4da86d5811a03c171a000001'), "a"=>"b", "c"=>"d"}

ree-1.8.7-2010.02 > result = coll.find_one
=>

{"_id"=>BSON::ObjectId('4da86d5811a03c171a000001'), "a"=>"b", "c"=>"d"}

ree-1.8.7-2010.02 > result.reject!

{ |key, value| key == 'a' }
=> {"_id"=>BSON::ObjectId('4da86d5811a03c171a000001'), "a"=>nil, "c"=>"d"}


Versus:


~ $ irb
>> a = { 'a' => 1, 'b' => 2 }
=> {"a"=>1, "b"=>2}
>> a.reject! { |key, value| key == 'a' }

=>

{"b"=>2}

>>

I've worked around this by using BSON::OrderedHash#delete_if