how to make a Recursive function in rails,that will return all CHILDREN of a parent,and children of all CHILDREN AND SO ON....TO N-LEVELS

On modern RDBs, recursive CTE can be used to handle recursive structure.

class Page < ApplicationRecord; end

Page.find_by_sql(
  "WITH RECURSIVE r AS (
     #{Page.where(id: 2).to_sql}
     UNION ALL
     #{Page.joins('JOIN r').where('r.id = pages.parent_id').to_sql})
   SELECT * FROM r")

#  Page Load (0.7ms)  WITH RECURSIVE r AS (
#     SELECT `pages`.* FROM `pages` WHERE `pages`.`id` = 2
#     UNION ALL
#     SELECT `pages`.* FROM `pages` JOIN r WHERE (r.id = pages.parent_id))                                                              
#   SELECT * FROM r
#    => [#<Page id: 2, parent_id: 1, created_at: "2018-08-21 15:00:43", updated_at: "2018-08-21 15:00:43">, #<Page id: 3, parent_id: 2, created_at: "2018-08-21 15:00:50", updated_at: "2018-08-21 15:00:50">]

As far as I know, mysql, postgres, sqlite3 supports recursive CTE.

---

EDIT@2020/12/17

On postgresql, you need to have following:

Page.find_by_sql(
  "WITH RECURSIVE r AS (
     #{Page.where(id: 2).to_sql}
   UNION ALL
     #{Page.joins('JOIN r ON r.id = pages.parent_id').to_sql})
   SELECT * FROM r")

(Thanks to @Dan for pointing out)

Here is some sample (it is not tested! But it will give you some clue):

def all_children(children_array = [])
  children = Page.where(parent_id: self.id)
  children_array += children.all
  children.each do |child|
    child.all_children(children_array)
  end
  children_array
end

So this is recursion that will try to find all children (nested too) from parent. I know that it is very ugly and unefficient, but I hope ii will give you a clue about finding nested elements.