create_private_room.rb

class CreatePrivateRooms < ActiveRecord::Migration[5.0]
  def change
    create_table :private_rooms do |t|
      t.timestamps
      t.string :name
    end
  end
end

output.sql

Loading development environment (Rails 5.0.2)
2.4.1 :001 > PrivateRoom.create
   (0.2ms)  BEGIN
  SQL (1.3ms)  INSERT INTO "private_rooms" ("created_at", "updated_at") VALUES ($1, $2) RETURNING "id"  [["created_at", 2017-06-15 15:58:13 UTC], ["updated_at", 2017-06-15 15:58:13 UTC]]
   (1.0ms)  COMMIT
 => #<PrivateRoom id: 2, created_at: "2017-06-15 15:58:13", updated_at: "2017-06-15 15:58:13", name: nil>

private_room.rb

class PrivateRoom < ApplicationRecord
  before_create :generate_link

  def to_params
    binding.pry
    self.name
  end

  private

  def generate_link
    binding.pry
    self.name = SecureRandom.hex(8).to_s
  end
end