gistfile1.txt

Problem: With a form_for in Rails 3, it checks object.errors['field'].any? 
which in DataMapper returns nil if no errors are found, 
in ActiveRecord it returns an empty array.

<% form_for @user, :url => "/users" do |f| %>
  <%= f.text_field :login %>
<% end %>

You have a nil object when you didn't expect it!
You might have expected an instance of Array.
The error occurred while evaluating nil.any?

Error at actionpack/lib/action_view/helpers/active_model_helper.rb:285
if object.respond_to?(:errors) && object.errors.respond_to?(:full_messages) && object.errors[@method_name].any?

ActiveRecord:
>> a = Post.new
=> #<Post id: nil, title: nil>
>> a.errors['title']
=> []
>> a.errors['title'].any?
=> false

DataMapper
>> a = User.new
=> #<User @id=nil @login=nil @encrypted_password=nil>
>> a.errors['login']
=> nil
>> a.errors['login'].any?
NoMethodError: You have a nil object when you didn't expect it!
You might have expected an instance of Array.
The error occurred while evaluating nil.any?
	from (irb):10