Primary key mode: pass find() a single primary key, and it will return a
single object matching that primary key.
If you pass findOne() no arguments, the first object of this type will be
returned.
If you pass findOne() an associative array, it will be turned into a
Horde_Rdo_Query object.
If you pass findOne() a Horde_Rdo_Query, it will return the first object
matching that query.