Naive uniqueness validator. If enabled, prevents adding a new record that has the same attribute
if one already exists in the corresponding table.
Critique: in a high-load system, it might be possible to create duplicate records due to bad timing
(another thread inserted a record after you tested for uniqueness, but before you inserted).
Suggestion: Use for small non-critical project. For real world projects it is recommended to
maintain uniqueness with database indexes and unique keys.