Millisecond resolution of DateTime in Ruby

ActiveRecord should preserve the full precision from the database, you're just not looking at it properly. Use strftime and the %N format to see the fractional seconds. For example, psql says this:

=> select created_at from models where id = 1;         created_at         ---------------------------- 2012-02-07 07:36:20.949641(1 row)

and ActiveRecord says this:

> Model.find(1).created_at.strftime('%Y-%m-%d %H:%M:%S.%N') => "2012-02-07 07:36:20.949641000" 

So everything is there, you just need to know how to see it.

Also note that ActiveRecord will probably give you ActiveSupport::TimeWithZone objects rather than DateTime objects but DateTime preserves everything too:

> '2012-12-31T01:01:01.232323+3'.to_datetime.strftime('%Y-%m-%d %H:%M:%S.%N') => "2012-12-31 01:01:01.232323000" 

Have a look at connection_adapters/column.rb in the ActiveRecord source and check what the string_to_time method does. Your string would go down the fallback_string_to_time path and that preserves fractional seconds as near as I can tell. Something strange could be going on elsewhere, I wouldn't be surprised given the strange things I've seen in the Rails source, especially the database side of things. I'd try converting the strings to objects by hand so that ActiveRecord will keeps its hands off them.

Changing m.happened_at = '2012-01-01T00:00:00.32323'.to_datetime in the code above to m.happened_at = '2012-01-01T00:00:00.32323' solves the problem, though I have no idea why.

I ended up here when I was suffering from using the RVM provided binary Ruby 2.0.0-p247 on OS X (Mavericks) which was causing rounding to whole values of seconds when retrieving times from Postgres. Rebuilding Ruby myself (rvm reinstall 2.0.0 --disable-binary) solved the issue for me.

See https://github.com/wayneeseguin/rvm/issues/2189 which I found via https://github.com/rails/rails/issues/12422.

I recognise that this is not THE answer to this issue but I hope this note might help someone struggling with it.