在Rails 4中的自定义验证期间，如何将date_select哈希转换为Date对象？

我是Rails和Ruby的新手。我目前正在一个个人项目中，在允许用户注册之前，我必须检查该用户是否至少13岁。为此，我使用了一种自定义验证方法，称为check_age_is_above_13。我遇到的问题是date_select视图中的助手将日期分为dob(1i)，dob(2i)和dob(3i)，并且我无法弄清楚如何在User模型中访问这些日期。我的代码如下：

移民：

class CreateUsers < ActiveRecord::Migration
  def change
    create_table :users do |t|
      t.string :email, null: false
      t.string :encrypted_password, null: false
      t.string :first_name
      t.string :last_name
      t.string :gender
      t.datetime :dob #date of birth
      t.timestamps
    end
  end
end

该User模型如下所示。我下面的代码片段发现在这里，尽管我必须承认这是一个有点..... OUT-日期：

class User < ActiveRecord::Base
  #using Devise for Auth
  devise :database_authenticatable, :registerable,
         :recoverable, :rememberable, :validatable, :trackable,
         :confirmable

  validates :gender, inclusion: { in: %w(female male unknown) }
  validate :check_age_is_above_13

  def check_age_is_above_13
    #The following raises "undefined local variable or method `params'" exception
    dob = Date.new params[:user]['dob(1i)'].to_i, params[:user]['dob(2i)'].to_i, params[:user]['dob(3i)'].to_i
    #Trying the below code raises a "TypeError, expected numeric" exception,
    #which is understandable, as "dob" does not exist in the params list
    if ((Date.today - dob) / 365.25).to_i < 13
      errors.add(:dob, "You must be at least 13 years old to register with us")
    end
  end
end

张贴注册表格时的参数如下所示：

{"utf8"=>"✓",
    "authenticity_token"=>"49w4ylxRIDyuRTDASW+vsuFL6Q3eeWcyokKrNXt9taM=",
    "user"=>{"email"=>"[email protected]",
        "password"=>"[FILTERED]",
        "password_confirmation"=>"[FILTERED]",
        "first_name"=>"Benny",
        "last_name"=>"Lava",
        "gender"=>"male",
        "location_id"=>"1",
        "dob(1i)"=>"2001",
        "dob(2i)"=>"8",
        "dob(3i)"=>"27"},
    "commit"=>"Sign up"}

dob(Xi)在模型上Date执行自定义验证之前，如何访问模型中的参数，以便可以先将其转换为对象？我正在使用Devise进行用户身份验证和注册，如果有什么不同的话。

编辑：和视图看起来像这样：

<h2>Sign Up</h2>
<%= form_for(resource, as: resource_name, url: registration_path(resource_name)) do |f| %>
  <%= devise_error_messages! %>
  <div>
    <%= f.label :email %><br />
    <%= f.email_field :email, autofocus: true %>
  </div>
  <div>
    <%= f.label :password %><br />
    <%= f.password_field :password, autocomplete: "off" %>
  </div>
  <div>
    <%= f.label :password_confirmation %><br />
    <%= f.password_field :password_confirmation, autocomplete: "off" %>
  </div>
  <div>
    <%= f.label :first_name %><br/>
    <%= f.text_field :first_name %>
  </div>
  <div>
    <%= f.label :last_name %><br/>
    <%= f.text_field :last_name %>
  </div>
  <div>
    <%= f.label :gender %><br/>
    <%= f.select :gender, options_for_select(%w[female male -]) %>
  </div>
  <div>
    <%= f.label :location_id %><br/>
    <%= f.select :location_id, options_for_select(Location.all.collect { |l| [l.name, l.id]}), {:include_blank => false} %>
  </div>
  <div>
    <%= f.label :dob, "Date of Birth" %><br/>
    <%= f.date_select :dob, {include_blank: false, start_year: Date.today.year - 100, end_year: Date.today.year - 13, default: {day: 31, month: 1, year: 1991}} %>
  </div>
  <div><%= f.submit "Sign up" %></div>
<% end %>

<%= render "devise/shared/links" %>