Module ActiveSupport::CachingTools::HashCaching
In: vendor/rails/activesupport/lib/active_support/caching_tools.rb

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

Methods

Public Instance methods

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]

    # File vendor/rails/activesupport/lib/active_support/caching_tools.rb, line 31
31:       def hash_cache(method_name, options = {})
32:         selector = options[:as] || "#{method_name}_cache"
33:         method = self.instance_method(method_name)
34:         
35:         args = []
36:         code = "def #{selector}(); @#{selector} ||= "
37:         
38:         (1..method.arity).each do |n|
39:           args << "v#{n}"
40:           code << "Hash.new {|h#{n}, v#{n}| h#{n}[v#{n}] = "
41:         end
42:         
43:         # Add the method call with arguments, followed by closing braces and end.
44:         code << "#{method_name}(#{args * ', '}) #{'}' * method.arity} end"
45:         
46:         # Extract the line number information from the caller. Exceptions arising
47:         # in the generated code should point to the +hash_cache :...+ line.
48:         if caller[0] && /^(.*):(\d+)$/ =~ caller[0]
49:           file, line_number = $1, $2.to_i
50:         else # We can't give good trackback info; fallback to this line:
51:           file, line_number = __FILE__, __LINE__
52:         end
53:         
54:         # We use eval rather than building proc's because it allows us to avoid
55:         # linking the Hash's to this method's binding. Experience has shown that
56:         # doing so can cause obtuse memory leaks.
57:         class_eval code, file, line_number
58:       end

[Validate]