Mongoid, references_one, and HTML select

Here's a problem I'm having on a Rails 3 app...

I have a MedicalProfessional model th开发者_C百科at looks kind of like this:

class MedicalProfessional
 include Mongoid::Document
 include Mongoid::Timestamps
 field :name, :type => String
 references_one :medical_specialty
end

and a MedicalSpecialty model that looks like this:

class MedicalSpecialty
 include Mongoid::Document
 field :name, :type => String
 validates_presence_of :name
 referenced_in :medical_professional
end

In a view, I create an HTML select like so:

select("medical_professional", "medical_specialty", @specialties)

When posting that form, I'm getting the following error:

NoMethodError in Medical_professionals#create

Showing new.html.haml where line #27 raised:

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.map

Line #27 is the one where I have the "select".

Any idea what the issue here is?

Your form post is failing, but the failed code path does not assign @specialties.