Rails call custom validation before .new or .create - ruby-on-rails

Question

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'] }

Answer 1

Sorry but that isn't even close to how you approach the problem in Rails.
If you want a user to have many pets and accept input for the pets when creating users you need to create a working assocation to a Pet model and have the User accept nested attributes:
class User < ApplicationRecord
has_many :pets # has_many assocations should always be plural!
validates :name, presence: true
validates :pets, presence: true
accepts_nested_attributes_for :pets
end
# rails g model pet name user:reference
class Pet < ApplicationRecord
belongs_to :user
validates :name, presence: true
end
class UsersController < ApplicationController
def new
#user = User.new(user_params)
3.times { #user.pets.new } # seeds the form with blank inputs
end
def create
#user = User.new(user_params)
if #user.save
redirect_to #user,
success: 'User created',
status: :created
else
render :new,
status: :unprocessable_entity
end
end
private
def user_params
params.require(:user)
.permit(:name, pets_attributes: [:name])
end
end
<%= form_with(model: #user) do |form| %>
<div class="field">
<%= form.label :name %>
<%= form.text_input :name %>
</div>
<fieldset>
<legend>Pets</legend>
<%= form.fields_for(:pets) do |pet_fields| %>
<div class="nested-fieldset">
<div class="field">
<%= pet_fields.label :name %>
<%= pet_fields.text_input :name %>
</div>
</div>
<% end %>
</fieldset>
<div class="actions">
<%= f.submit %>
</div>
<% end %>
This is a pretty advanced topic and you should have your Rails CRUD basics well figured out before you attempt it. You should also consider if you instead want to use a separate controller to create the pets one by one as a nested resource after creating the user.