Provide shortcuts to simply the creation of nested default hashes. This pattern is useful, common practice, and unsightly when done manually.

Methods
H
Instance Public methods
hash_cache(method_name, options = {})

Dynamically create a nested hash structure used to cache calls to method_name The cache method is named +#{method_name}_cache+ unless :as => :alternate_name is given.

The hash structure is created using nested Hash.new. For example:

def slow_method(a, b) a ** b end

can be cached using #hash_cache :slow_method, which will define the method slow_method_cache. We can then find the result of a ** b using:

slow_method_cache[a][b]

The hash structure returned by slow_method_cache would look like this:

Hash.new do |as, a|
  as[a] = Hash.new do |bs, b|
    bs[b] = slow_method(a, b)
  end
end

The generated code is actually compressed onto a single line to maintain sensible backtrace signatures.

Source: show 

# File rails/activesupport/lib/active_support/caching_tools.rb, line 31
def hash_cache(method_name, options = {})
  selector = options[:as] || "#{method_name}_cache"
  method = self.instance_method(method_name)
  
  args = []
  code = "def #{selector}(); @#{selector} ||= "
  
  (1..method.arity).each do |n|
    args << "v#{n}"
    code << "Hash.new {|h#{n}, v#{n}| h#{n}[v#{n}] = "
  end
  
  # Add the method call with arguments, followed by closing braces and end.
  code << "#{method_name}(#{args * ', '}) #{'}' * method.arity} end"
  
  # Extract the line number information from the caller. Exceptions arising
  # in the generated code should point to the +hash_cache :...+ line.
  if caller[0] && %r^(.*):(\d+)$/ =~ caller[0]
    file, line_number = $1, $2.to_i
  else # We can't give good trackback info; fallback to this line:
    file, line_number = __FILE__, __LINE__
  end
  
  # We use eval rather than building proc's because it allows us to avoid
  # linking the Hash's to this method's binding. Experience has shown that
  # doing so can cause obtuse memory leaks.
  class_eval code, file, line_number
end