nested_attributes_for

There is a lot more going on with my models, but I simplified the class for this bug. I have a class, finding_data, which has many finding_measurement_datas. Everything works except for updating with the nested attributes. I assigned class

FindingData < ActiveRecord::Base has_many :finding_measurement_datas, :dependent => :destroy accepts_nested_attributes_for :finding_measurement_datas end

class FindingMeasurementData < ActiveRecord::Base

    belongs_to :finding_measurement
    belongs_to :finding_data
    validates_presence_of :finding_measurement

end

I send back params that look like this from my application to the server.

Parameters: {"id"=>"97", "exam_record"=>{"name"=>"exam_record", "finding_datas"=> [{"name"=>"Mammo Mass 1", "updated_at"=>"2009-03-27T21:54:24-05:00", "finding_type_id"=>"15", "selected"=>"1", "id"=>"1", "finding_measurement_datas"=> [{"finding_measurement_id"=>"15", "id"=>"1", "measurement"=>"1.34", finding_data_id"=>"1"}, {"finding_measurement_id"=>"19", "id"=>"2", "measurement"=>"1", "finding_data_id"=>"1"}], }}}

In the controller I say

params[:exam_record][:finding_datas].each do |fd|

      if FindingData.exists?(fd[:id])
        finding_data = FindingData.find(fd[:id]).update_attributes(fd)
      else
        finding_data = FindingData.create(fd)
      end

end

This should then take the list of finding_measurement_datas that is in fd and update the finding data, and add any associated measurements correct? Instead it tells me in the log

ActiveRecord::AssociationTypeMismatch (FindingMeasurementData(#-608683668) expected, got HashWithIndifferentAccess(#-605880508)): app/controllers/readings_controller.rb:193:in update'

I would just cast each measurement data as a new measurement data, but some aren't new and they already have ids.

I am trying to follow the documents about doing complex views. I am sending XML back and fourth to my server, but since rails parses it into params I assume it should work the same way as the params hash from the rails views. I also know that I can iterate through the hash and add each finding measurement on its own, the link it to the finding data. Should what I'm trying to do work, or am I not looking at the nested_attributes_for correctly?

Thanks for checking it out