Bidirectional Hash table in Ruby

I need a bidirectional Hash table in Ruby. For example:

h = {:abc => 123, :xyz => 789, :qaz => 789, :wsx => [888, 999]}
h.fetch(:xyz) # => 789
h.rfetch(123) # => abc
h.rfetch(789) # => [:xyz, :qaz]
h.rfetch(888) # => :wsx

Method rfetch means reversed fetch and is only my proposal.

Note three things:

1. If multiple keys map at the same value then rfetch returns all of them, packed in array.
2. If value is an array then rfetch looks for its param among elements of the array.
3. Bidirectional Hash means that both fetch and rfetch should execute in constant time.

Does such structure exists in Ruby (including external libraries)?

I thought about implementing it using two one-directional Hashes synchronized when one of them is modified (and packing it into class to avoid synchronization problems) but maybe I could use an already existing solution?