Question

I have a model that loads dynamically data from a REST API if a property is accessed and its property has no data:

class Issue
  attr_accessor :ticket_title, :priority, :description
  @ticket_title
  @priority
  @description

  def priority
    if !@priority.empty?
      updateProperties()
    end
    @priority
  end
  def description
    if !@description.empty?
      updateProperties()
    end
    @description
  end
  def ticket_title
    if !@ticket_title.empty?
      updateProperties()
    end
    @ticket_title
  end

  def updateProperties
    # loads all data from REST API
  end

  def initialize (hsh = {})
      hsh.each { |key, value|
        self.instance_variable_set("@#{key}", value) 
      }
  end

end

There are two problems:

1. If I initialize the model with RedmineIssue.new :ticket_title => 'test', I dont want to the model to call updateProperties, but somehow it does.
2. Is there a way to declare functions magically? I.e., if there is no real function try to run a meta function?

Answer 1

Well, here’s something for the “magic”. In your class, if you want to be able to get any part of a hash through a method named get_whateverthekeyis (silly example, I know), you’d use method_missing:

class Foo
  def initialize
    @h = {}
  end
  def method_missing(name, *args, &block)
    if(name =~ /\Aget_(.+)\Z/)
      #Name is valid
      sig_class.class_exec($1) do |n|
        define_method("get_"+n) {@h[n]}
      end

      @h[$1]
    elsif(name =~ /\Aset_(.+)\Z/)
      #Name is valid
      sig_class.class_exec($1) do |n|
        define_method("set_"+n) {|v| @h[n]=v}
      end

      @h[$1] = args[0]
    else
      super
    end
  end
  private
  def sig_class
    class << self
      self
    end
  end
end

When the method is not defined, it goes to method_missing. When that happens, method_missing does the requested operation on the hash, and then dynamically defines the method, so that next time it is a direct method call, meaning that it is much more efficient because the method already exists. This is somewhat akin the the method_missing used by Ruby’s own OpenStruct.