upload.rb

# Potential solution to "Using accepts_nested_attributes_for with a belongs_to association, and using find_or_create_by_attr behaviour"
# http://stackoverflow.com/questions/2970255/using-accepts-nested-attributes-for-with-a-belongs-to-association-and-using-find
class Upload < AR:B
  belongs_to :user
  belongs_to :observed_property
  belongs_to :sensor
  
  attr_accessor :observed_property_attributes,
                :sensor_attributes
                
  before_validation :update_associations, unless: "@observed_property_attributes.nil? || @sensor_attributes.nil?"

  validate :validate_associations

  def update_associations
    observed_property = ObservedProperty.find_by(name: @observed_property_attributes[:name])
    if observed_property.nil?
      observed_property = ObservedProperty.new(@observed_property_attributes)
    end
    self.observed_property = observed_property
    
    sensor = Sensor.find_by(name: @sensor_attributes[:name])
    if sensor.nil?
      sensor = Sensor.new(@sensor_attributes)
    end
    self.sensor = sensor
  end
  
  def validate_associations
    merge_errors(observed_property) unless self.observed_property.valid?
    merge_errors(sensor) unless self.sensor.valid?
  end
  
  def merge_errors(instance)
    instance.errors.each do |att, msg|
      if att == 'base'
        errors.add("#{instance.class.to_s.downcase}_attributes", msg)
      else
        errors.add("#{instance.class.to_s.downcase}_attributes_#{att}", msg)
      end
    end
  end
  
end

upload_spec.rb

# Matching spec
# spec_helper.rb defines a few sample users
require 'spec_helper'

describe Upload do
  before(:each) do
    @valid_attributes = {
      :observed_property_attributes => Factory.attributes_for(:observed_property, :creator => @user_a),
      :sensor_attributes => Factory.attributes_for(:sensor, :creator => @user_a)
    }
  end
  
  it "should create a new instance given valid attributes" do
    upload = @user_a.uploads.create!(@valid_attributes)
    upload.observed_property.should_not be_nil
    upload.sensor.should_not be_nil
  end
  
  it "should use existing models when linked to them, regardless of creator" do
    observed_property = Factory :observed_property, :creator => @user_b
    sensor = Factory :sensor, :creator => @user_c
    upload = @user_a.uploads.create!(
        :observed_property_attributes => { :_id => observed_property._id },
        :sensor_attributes => { :_id => sensor._id }
        )
    upload.observed_property.should == observed_property
    upload.sensor.should == sensor
  end
  
  it "should not save with blank attributes" do
    upload = @user_a.uploads.new
    upload.observed_property = ObservedProperty.new
    upload.sensor = Sensor.new
    upload.update_attributes(
        :observed_property_attributes => {
            :_id => "",
            :unit => "",
            :description => ""
            },
        :sensor_attributes => {
            :_id => "",
            :description => ""
            }
        )
    lambda {
      upload.save!
    }.should raise_error
  end
end