gistfile1.ru

# ruby 1.8.7

# Hash implicitly creates an array for empty nodes
a = Hash.new([])
a[:key] << 2 # => [2]
p a # => {} nil
p a[:key] # => [2]

# Explicitly add an array for all nodes before creating
b = Hash.new
b[:key] ||= []
b[:key] << 2 # => [2]
p b # => {:key=>[2]}