Unique Constraints feature was first introduced in db4o 6.2.
Unique Constraints allow a user to define a field
to be unique across all the objects of a particular Class stored to db4o. This
means that you cannot save an object where a previously committed object
has the same field value for fields marked as unique.
A Unique Constraint is checked at commit-time and a constraint violation will cause a UniqueFieldValueConstraintViolationException to be thrown. This functionality is based on Commit-Time Callbacks feature.