Few days back, I was working on a project where I have to create nested comments on a articles and on showing the article I was needed to show all the nested comments in its hierarchy.So I used closure_tree gem to achieve that. So lets see how I implemented the nested comment functionality on article.

1) Add the following Gem to your Gemfile.

gem 'closure_tree'

2) Run the bundle to install the Gem

bundle

3) Now create the comments model which will have article_id (in case of root comment which will be posted directly on the article), parent_id (for storing the parent comment id), user_id, text (to store the content of the text)

rails g model Comment article:references parent_id:integer text:string

4) Now run the following migration for getting nested comments hierarchy

rails g migration create_comment_hierarchies

5) Inside this migration write the following code

class CreateCommentHierarchies < ActiveRecord::Migration
  def change
    create_table :comment_hierarchies, :id => false do |t|
      t.integer  :ancestor_id, :null => false   # ID of the parent/grandparent/great-grandparent/... comments
      t.integer  :descendant_id, :null => false # ID of the target comment
      t.integer  :generations, :null => false   # Number of generations between the ancestor and the descendant. Parent/child = 1, for example.
    end

    # For "all progeny of" and leaf selects:
    add_index :comment_hierarchies, [:ancestor_id, :descendant_id, :generations],
              :unique => true, :name => "comment_anc_desc_udx"

    # For "all ancestors of" selects,
    add_index :comment_hierarchies, [:descendant_id],
              :name => "comment_desc_idx"
  end
end

6) Inside Comment model write this line

acts_as_tree order: 'created_at DESC'
belongs_to :parent, class_name: "Comment"
has_many :childrens, class_name: "Comment", foreign_key: "parent_id"

7) Now during creation of the comment you need to create the comment like this:

def create
  if params[:comment][:parent_id].present?
    parent = Comment.where(params[:comment][:parent_id]).first
    comment = parent.children.build(comment_params)
  else
    comment = Comment.new(comment_params)
    if comment.save
   	render json: {success: true, message: "Comment Created Successfully"}
    else
     render json: {success: false, message: "Something went wrong"}
    end
 end
end

8) Now to get the hierarchy of a comment you just need to write like this:

Comment.hash_tree

 and it will give you a result set like this:

 {"k1l1" =>
	{"k1l2" =>
	   {"k1l3" =>
	     {"k1l4" => {}
	   },
	   "k2l3" =>
	     {"k2l4" => {}}
	   },
	   "k3l3" => {}
	}
}

You can also define the depth limit of the tree:

Comment.hash_tree(limit_depth: 2)