如何在Rails中验证表单实例的唯一性，而不是整个数据库的唯一性？

class User < ApplicationRecord
  validates :email, uniqueness: true
end

class UsersController < ApplicationController
  def create
    @user = User.new(user_params)
    if @user.save
      # 用户保存成功的逻辑
    else
      # 用户保存失败的逻辑
    end
  end

  private

  def user_params
    params.require(:user).permit(:email, :password, :password_confirmation)
  end
end

<%= form_for @user do |f| %>
  <%= f.label :email %>
  <%= f.text_field :email %>
  <% if @user.errors.has_key?(:email) %>
    <div class="error">
      <%= @user.errors[:email].first %>
    </div>
  <% end %>

  <%= f.label :password %>
  <%= f.password_field :password %>

  <%= f.label :password_confirmation %>
  <%= f.password_field :password_confirmation %>

  <%= f.submit "注册" %>
<% end %>