entries table
id | parent_id 

class Entry < ApplicationRecord
  belongs_to :parent, class_name: "Entry"
  has_many :children, class_name: "Entry"

Entry.last.children.last.parent

on different column name:

entries table
id | parent_entry_id 

class Entry < ApplicationRecord
  belongs_to :parent, class_name: "Entry", foreign_key: :parent_entry_id   #, optional: true
  has_many :children, foreign_key: :parent_entry_id,  class_name: "Entry"

Entry.last.children.last.parent