Rails call custom validation before .new or .create

I make objects in controller's loop.

I need to check pet_name array before loop starts. (because i got undefined method 'each' for nil:NilClass when params[:pet_name].each do |pid|) runs)

But my validation always called after User.new or User.create. I want to change to validate as when i push submit button and check validation, and redirects back when pet_name array is nil. Ho can i change my code?

Controller

  def create
    user_name = params[:user_name]
    
    params[:pet_name].each do |pid|
      @user = User.new
      @user.name = user_name
      @user.pet_name = pid
      render :new unless @user.save
    end
    redirect_to users_path
  end

User.rb

class User < ApplicationRecord
  has_many :pet

  validates :name, presence: true
  validates :pet_name, presence: true
  validate :check_pet
  def check_pet
    if pet_name.nil?
      errors.add(:pet_name, 'at least one pet id')
    end
  end
end

Prams structure

  { name: 'blabla', pet_name: ['blabla', 'blablabla', 'bla'] }