A little gotcha with custom setters for datetime attributes is, that when setting an attribute through ‘write_attribute’ it is not converted to UTC (or whatever else your default time zone is).
This problem can easily be reproduced:

./script/generate model Thing a:datetime b:datetime

My model:

class Thing < ActiveRecord::Base
  def b=(d)
    write_attribute(:b, d)
  end
end

My tests:

require 'test_helper'
 
class ThingTest < ActiveSupport::TestCase
  test "a and b" do
    d = DateTime.now
    t = Thing.new
    t.a = d
    t.b = d
    t.save
    assert_equal t.a, t.b
    t.reload
    assert_equal t.a, t.b
  end
end

The second assertion will fail. Is this intended or a bug?
Quick fix:

  def b=(d)
    d = d.utc
    write_attribute(:b, d)
  end

I included jQuery by installing the jRails plugin, which seems nice because you can continue using rails’ ajax helpers. But when I tried to implement a more special functionality by using the $.ajax function it proved impossible to set the Accept header for my request. Neither using the dataType option nor by setting it directly in a beforeSend function. The Accept header always read:

text/javascript, text/html, application/xml, text/xml, */*

I was about to uninstall and hate jQuery for the rest of my life.
By coincidence I took a look at jrails.js which is a part of the jRails plugin:

(function($)
{
  $().ajaxSend(
    function(a,xhr,s){
      xhr.setRequestHeader("Accept","text/javascript, text/html, application/xml, text/xml, */*")
    }
  )
}
)(jQuery); 
[...]

WTF? Is that documented anywhere?
This hard coded shit stuff breaks rails’ respond_to functionality, doesn’t it?t
To be fair, it works if you use extensions to determine what datatype you expect. (Like /things/1.js)
But I don’t do that when I build a custom ajax request where I can set the Accept header directly.

Update:
Some research on the topic “accept header vs. extension” showed that in terms of cross browser compatibility I should favor the extension approach. And I’m not surprised it’s Microsoft’s fault

It’s somehow trivial, but I found out about ‘write_attribute‘ only just: [another case of rtfm]

class Thing < ActiveRecord::Base
  def name=(str)
    str.upcase! unless str.nil?
    write_attribute(:name, str)
  end
end

Update:
Be careful if you use this with Datetime attributes.