Nested comments from scratch

Question

Let\'s say I have a comment model:

class Comment < ActiveRecord::Base
    has_many :replies, class: \"Comment\", foreign_key: \"reply_id\"
end
         


        

Answer 1

That can be solved with resursion or with a special data structure. Recursion is simpler to implement, whereas a datastructure like the one used by the nested_set gem is more performant.

Recursion

First an example how it works in pure Ruby.

class Comment < Struct.new(:content, :replies); 

  def print_nested(level = 0)
    puts "#{'  ' * level}#{content}"   # handle current comment

    if replies
      replies.each do |reply|
        # here is the list of all nested replies generated, do not care 
        # about how deep the subtree is, cause recursion...
        reply.print_nested(level + 1)
      end
    end
  end
end

Example

comments = [ Comment.new(:c_1, [ Comment.new(:c_1a) ]),
             Comment.new(:c_2, [ Comment.new(:c_2a),
                                 Comment.new(:c_2b, [ Comment.new(:c_2bi),
                                                      Comment.new(:c_2bii) ]),
                                 Comment.new(:c_2c) ]),
             Comment.new(:c_3),
             Comment.new(:c_4) ]

comments.each(&:print_nested)

# Output
# 
# c_1
#   c_1a
# c_2
#   c_2a
#   c_2b
#     c_2bi
#     c_2bii
#   c_2c
# c_3
# c_4

And now with recursive calls of Rails view partials:

# in your comment show view 
<%= render :partial => 'nested_comment', :collection => @comment.replies %>

# recursion in a comments/_nested_comment.html.erb partial
<%= nested_comment.content %>
<%= render :partial => 'nested_comment', :collection => nested_comment.replies %>

Nested Set

Setup your database structure, see the docs: http://rubydoc.info/gems/nested_set/1.7.1/frames That add the something like following (untested) to your app.

# in model
acts_as_nested_set

# in controller
def index
  @comment = Comment.root   # `root` is provided by the gem
end

# in helper
module NestedSetHelper

  def root_node(node, &block)
    content_tag(:li, :id => "node_#{node.id}") do
      node_tag(node) +
      with_output_buffer(&block)
    end
  end

  def render_tree(hash, options = {}, &block)
    if hash.present?
      content_tag :ul, options do
        hash.each do |node, child|
          block.call node, render_tree(child, &block)
        end
      end
    end
  end

  def node_tag(node)
    content_tag(:div, node.content)
  end

end

# in index view

  <%= render 'tree', :root => @comment %>


# in _tree view
<%= root_node(root) do %>
  <%= render_tree root.descendants.arrange do |node, child| %>

    <%= content_tag :li, :id => "node_#{node.id}" do %>
      <%= node_tag(node) %>
      <%= child %>
    <% end %>

  <% end %>
<% end %>

This code is from an old Rails 3.0 app, slightly change and untested. Therefore it will probably not work out of the box, but should illustrate the idea.