Metaprogramming section

[bbatsov]

I'm about to introduce a new section devoted to metaprogramming. This issue is the place to share your opinion about good and bad practices that you want to see listed in the new section.

[mrflip]

• The block form of class_eval is preferable to the string-interpolated form.

  • when you use the string-interpolated form, always supply __FILE__ and __LINE__, so that your backtraces make sense:

    class_eval "def use_relative_model_naming?; true; end", __FILE__, __LINE__

  • define_method is preferable to class_eval{ def ... }

• When using class_eval (or other eval) with string interpolation, add a comment block showing its appearance if interpolated (a practice I learned from the rails code):

  # from activesupport/lib/active_support/core_ext/string/output_safety.rb
  UNSAFE_STRING_METHODS.each do |unsafe_method|
    if 'String'.respond_to?(unsafe_method)
      class_eval <<-EOT, __FILE__, __LINE__ + 1
        def #{unsafe_method}(*args, &block)       # def capitalize(*args, &block)
          to_str.#{unsafe_method}(*args, &block)  #   to_str.capitalize(*args, &block)
        end                                       # end
  
        def #{unsafe_method}!(*args)              # def capitalize!(*args)
          @dirty = true                           #   @dirty = true
          super                                   #   super
        end                                       # end
      EOT
    end
  end

• avoid using method_missing for metaprogramming. Backtraces become messy; the behavior is not listed in #methods; misspelled method calls might silently work (nukes.luanch_state = false). Consider using delegation, proxy, or define_method instead. If you must use method_missing,

  • be sure to also define respond_to?

  • only catch methods with a well-defined prefix, such as find_by_* -- make your code as assertive as possible.

  • call super at the end of your statement

  • delegate to assertive, non-magical methods:

    # bad
    def method_missing?(meth, *args, &block)
      if /^find_by_(?<prop>.*)/ =~ meth
        # ... lots of code to do a find_by
      else
        super
      end
    end
    
    # good
    def method_missing?(meth, *args, &block)
      if /^find_by_(?<prop>.*)/ =~ meth
        find_by(prop, *args, &block)
      else
        super
      end
    end
    
    # best of all, though, would to define_method as each findable attribute is declared  

[bbatsov]

Great suggestions!