Question

Imagine I have an object model:

A Blog has many Articles, and an Article has many Comments

Imagine also that I have two blogs, Blog A and Blog B.

Blog A - Article id 1 - Comment id 1 "fun stuff"
       - Article id 2 - Comment id 2 "cool"

and

Blog B - Article id 3 - Comment id 3 "no fun"

I need to compare the object graph for Blog A and Blog B, and update Blog B based on the value of objects in Blog A.

In this case, Blog B should change Comment 3 to be “fun stuff”, and instantiate new objects with values identical to Article 2 and Comment 2.

Recursively walking the graph is the obvious solution, but the logic gets convoluted. I’d rather not re-invent the wheel…is there a pattern or process to do this?

I’m using Ruby/Rails

Answer 1

After reading more about the visitor pattern, I decided a Rubyish variant of it was the most appropriate approach to solving this problem.

The visitor pattern allows you to separate the algorithm for walking the hierarchy, from the code to execute on each node in the hierarchy. A more functional approach to this using map or inject/fold is possible…but as I want to reuse the operators, it seemed easier to break them into separate classes.

The hierarchy is implemented in each model, which should define a “children” method that returns children.

Below is my implementation, based off of various references, I may wrap it up into a gem.

module Visitable
  def accept visitor
    child_vals = []
    if respond_to?(:children)
      children.each do |child|
        child_vals << child.accept(visitor)
      end
    end
    val = visitor.visit(self)
    child_vals.any? ? val + child_vals : val
  end
end

class Survey
  attr_accessor :name, :children

  include Visitable

end

class Category
  attr_accessor :name, :children

  include Visitable

end

class Question
  attr_accessor :name
  include Visitable
end

s = Survey.new
s.name = 's1'
c = Category.new
c.name = 'c1'
c2 = Category.new
c2.name = 'c2'
q = Question.new
q.name = 'q1'
q2 = Question.new
q2.name = 'q2'

c.children = [q]
c2.children = [q2]
s.children = [c,c2]

class ReturnVisitor
  def visit obj
    obj.name
  end
end

s.accept(ReturnVistor.new)
-> ['s1', ['c1', ['q1'], ['c2', ['q2']]]]

# "poorly implemented lisp"?