Add comment model and its tests

app/models/comment.rb

@@ -0,0 +1,8 @@
+class Comment < ApplicationRecord
+  belongs_to :user
+  belongs_to :post
+  belongs_to :parent, class_name: 'Comment', optional: true
+  has_many :children, class_name: 'Comment', foreign_key: 'parent_id', dependent: :destroy
+
+  validates :body, presence: true, length: { minimum: 4 }
+end