Validates nested fields if another field is blank in rails form

I have Product model with price field and product_prices nested fields. I would validate the presence at least one of two and I would reject the presence of nested fields if price field is not blank. Here my Product model:

class Product < ApplicationRecord

  belongs_to :model
  belongs_to :category
  belongs_to :sub_category
  validates :name, :model_id, :image, presence: true
  validates :name, uniqueness: true
  has_many :product_prices
  accepts_nested_attributes_for :product_prices, :allow_destroy => true

  mount_uploader :image, ImageUploader
end

# == Schema Information
#
# Table name: products
#
#  id              :bigint(8)        not null, primary key
#  name            :string(255)
#  model_id        :integer
#  category_id     :integer
#  sub_category_id :integer
#  image           :string(255)
#  price           :float(24)
#  created_at      :datetime         not null
#  updated_at      :datetime         not null
#  user_id         :integer

and my ProductPrice model:

class ProductPrice < ApplicationRecord

  belongs_to :product

end

# == Schema Information
#
# Table name: product_prices
#
#  id              :bigint(8)        not null, primary key
#  product_id      :integer
#  from            :integer
#  to              :integer
#  price           :float(24)
#  created_at      :datetime         not null
#  updated_at      :datetime         not null

Someone can help me please?

You can do with with a custom validation method.

class Product < ApplicationRecord
  has_many :product_prices, optional: true
  validate :need_price

  private

  def need_price
    if price.present?
      errors.add(:price, 'not allowed if a product price is given') if product_prices.present?
    else 
      errors.add(:price, 'need price or add a product price') if product_prices.empty?
    end
  end
end