Efficient way to display a nested tree with mongoid and rails

Question

I've got a comment tree nested in the document, using mongoid embeds_many_recursively like this:

   Document: {
    ...
    comments: [{
      ...
      updated_at,
      child_comments: [{
        ...
        updated_at
        child_comments: [{...},{...}],
        ...},{...}]
      ...}]
    ...}]
   ...}

What's the most effective way of passing it to a view in a way that is ordered by first level 'comment updated_at' attribute?

At the moment I came up with this inside the main document model:

  def flatten_comments
    @flat_comments = []
    self.comments.order_by([[:updated_at, :desc]]).each do |comment|
      flatten_comments_iterator(comment)
    end
    return @flat_comments
  end

  def flatten_comments_iterator(comment)
    @flat_comments << comment
    comment.child_comments.each {|reply| flatten_comments_iterator(reply)}
  end

and then just iterating in the view over the array.

The problems are: 1) in the recursive flattening, the order is lost somewhere, and I can't figure where, step-by-step on paper it seems to adding items in the needed order, probably some thing to do with class variable scope and access.

2) I'm not sure it's the most efficient way to do a simple retrieval.

Would be thankful for an advise and for experience with how to handle this kind of tasks efficiently.

score 2 · Accepted Answer · edited May 23 '17 at 12:04

There are basically 2 design approaches (one of them being the same as yours), that are documented on ruby driver's modeling examples. There is also a similar question on SO about it.

About the other issue: there's nothing bad about recursivity, in general, if the comments don't have a huge depth of nesting. However your implementation is not thread-safe, as it uses instance variables, and not local variables. To deal with it, you should transform @flat_comments to a local variable and pass it as a param to flatten_comments_iterator method.

Tip: since any method recursion can be transformed to a iteration, what you may want to implement is an iterative preorder traversal of a graph.

Efficient way to display a nested tree with mongoid and rails

1 Answers1