I found a couple of incompatibilities between Arel and ActiveRecord where ActiveRecord handles a database structure but Arel doesn't so that calling MyModel.first works fine, but MyModel.find does not.
The first problem is with non-standard SQL column types like PostGIS geometry columns on a table.
http://rails.lighthouseapp.com/projects/8994/tickets/5477
The second problem is with mapping ActiveRecord classes to database views instead of tables: