migration.rb

class CreateUsers < ActiveRecord::Migration
  def change
    enable_extension 'uuid-ossp'
    create_table :users, id: false do |t|
      t.primary_key   :id, :uuid, :default => 'uuid_generate_v1()'
      t.string        :first_name
      t.string        :last_name
      t.string        :email
      t.string        :mobile_number
      t.string        :locality
      t.string        :city
      t.string        :address
      t.string        :blood_group
      t.string        :emeregency_number
      t.string        :user_type
      t.integer       :dob
      t.string        :password
      t.string        :access_token
      t.uuid          :parent_id
      t.timestamps
    end
  end
end

model.rb

class User < ActiveRecord::Base
  validates :first_name,:last_name,                   presence:   true
  validates :email,:mobile_number,                     presence:   true
  validates :locality,:city,                          presence:   true
  validates :email,                                   uniqueness: true
  
  belongs_to :parent,   :inverse_of => :children,     class_name: "User"
  has_many   :children, :inverse_of => :parent,       class_name: "User"
  
  before_create :generate_access_token

private
  
  def generate_access_token    
      access_token = SecureRandom.hex
      until User.find_by_access_token(access_token) == nil
        access_token = SecureRandom.hex
      end
      self.access_token = access_token
  end
end

query.rb

admin = User.find_by user_type: "admin"
admin.children                   # throws ERROR: column users.user_id does not exist

#########################################

franchisee = User.find_by user_type: "franchisee"
franchisee.parent                # WORKS!!!

seed.rb

admin = User.create! :first_name => 'admin', :last_name => 'admin', 
:email => 'admin@example.com', :mobile_number => '9999911111', 
:locality => 'Goregaon', :city => 'Mumbai', 
:user_type => 'admin', :password => 'admin'

franchisee = User.create! :first_name => 'franchisee', :last_name => 'franchisee', 
:email => 'franchisee@example.com', :mobile_number => '9999911111', 
:locality => 'Goregaon', :city => 'Mumbai', 
:user_type => 'franchisee', :password => 'franchisee'

franchisee.parent = admin
franchisee.save