Found a simpler/cleaner way to do this

Thankfully, no longer requires disgusting hacking into
devise. Instead just a tri-state boolean, that can never be false.*

* Simples.

app/models/user.rb

@@ -12,6 +12,8 @@ class User < ActiveRecord::Base
 
   before_save :ensure_authentication_token
 
+  validates :agreed_to_terms, acceptance: {accept: true}, allow_nil: true 
+
   def ensure_authentication_token
     if authentication_token.blank?
       self.authentication_token = generate_authentication_token
@@ -29,34 +31,6 @@ def after_password_reset
     confirm!
   end
 
-  # Coped from https://github.com/plataformatec/devise/blob/v3.0.3/lib/devise/models/database_authenticatable.rb
-  # Sorry but I couldn't add a check around this method as .valid? wipes
-  # other errors from the hash
-  def update_with_password(params, *options)
-    current_password = params.delete(:current_password)
-
-    if params[:password].blank?
-      params.delete(:password)
-      params.delete(:password_confirmation) if params[:password_confirmation].blank?
-    end
-
-    # special case for rails handling of boolean check_box
-    agreed_to_terms = params[:agreed_to_terms] == "1"
-
-    result = if valid_password?(current_password) && agreed_to_terms
-      update_attributes(params, *options)
-    else
-      self.assign_attributes(params, *options)
-      self.valid?
-      self.errors.add(:current_password, current_password.blank? ? :blank : :invalid) unless valid_password?(current_password)
-      self.errors.add(:agreed_to_terms, :blank) unless agreed_to_terms?
-      false
-    end
-
-    clean_up_passwords
-    result
-  end
-
   def to_s
     if name.present?
       "#{name} <#{email}>"

db/migrate/20150119114745_add_extra_user_details.rb

@@ -1,7 +1,7 @@
 class AddExtraUserDetails < ActiveRecord::Migration
   def up
     add_column :users, :organization, :string
-    add_column :users, :agreed_to_terms, :boolean, null: false, default: false
+    add_column :users, :agreed_to_terms, :boolean, null: true, default: nil
   end
 
   def down

db/schema.rb

@@ -417,7 +417,7 @@
     t.datetime "confirmation_sent_at"
     t.string   "unconfirmed_email"
     t.string   "organization"
-    t.boolean  "agreed_to_terms",        :default => false, :null => false
+    t.boolean  "agreed_to_terms"
   end
 
   add_index "users", ["authentication_token"], :name => "index_users_on_authentication_token", :unique => true

test/unit/user_test.rb

@@ -101,4 +101,16 @@ class UserTest < ActiveSupport::TestCase
     assert_equal "Joan Jett <joan@example.com>", user.to_s
   end
 
+  test "can create a user without specifying agreement to terms" do
+    assert User.new(email: 'robyn@example.com', password: 'password').valid?
+  end
+
+  test "can create a user that agreeds to terms" do
+    assert User.new(email: 'robyn@example.com', password: 'password', agreed_to_terms: true).valid?
+  end
+
+  test "not agreeing to terms prevents saving" do
+    refute User.new(email: 'robyn@example.com', password: 'password', agreed_to_terms: false).valid?
+  end
+
 end