recording.rb

class Recording < ActiveRecord::Base
    has_many :frames, :order => "time_ms"
end

recording_spec.rb

describe Recording do
    it 'should order frames properly' do
        recording = Recording.new
        frame3 = recording.frames.build :time_ms => 3
        frame1 = recording.frames.build :time_ms => 1
        frame2 = recording.frames.build :time_ms => 2
        recording.save.should == true

        recording.frames.should == [ frame1, frame2, frame3 ]
    end
end

rspec output (snippit)

'Recording should order frames properly' FAILED
expected: [#<Frame id: 2, recording_id: 1, time_ms: 1>, #<Frame id: 3, recording_id: 1, time_ms: 2>, #<Frame id: 1, recording_id: 1, time_ms: 3>],
     got: [#<Frame id: 1, recording_id: 1, time_ms: 3">, #<Frame id: 2, recording_id: 1, time_ms: 1>, #<Frame id: 3, recording_id: 1, time_ms: 2>] (using ==)

The solution was to add a recording.reload at recording_spec.rb:8 (to flush from memory).