#fields_for always expects a model

I have a case where I have a hash object as a property of a alternative-ORM model, which I want to make form fields for. Using the fields_for helper almost works, except in one case, where it attempts to call #new_record? on the hash. Attached is a patch to check for the method first, and it works great.

As an example, the model I'm trying to construct fields for is generated like this:

def build_metric_filter
  @metric_filter = MetricFilter.new
  @metric_filter.criteria = [{:target => "Hostname", :comparison => "is", :pattern => ""}]
  @metric_filter
end