We want the created_at and updated_at field support that ActiveRecord gives you out of the box. Since we are using Postgres (and after reading a number of threads on the topic in this group), we decided that the best way to populate each of these fields was via default values and a database trigger (updates).
One thing that would be ideal is to have 'read-only' columns. i.e. I'd like to be able to query these fields at will but not have to worry about them being overwritten by our model objects when doing updates. Having to manually code custom update functions to avoid this seems painful - is there a better approach?