Add unit tests for validation

**Feature request**
It would be great to have logging for models.

**Use case**
Many users need this functionality for production deployments.

**Implementation idea**
Add a new config option `enable_logging` with default `false`.

No files, tests, or entry points are named. Start by locating the validation and configuration handling for models, then determine where model logging belongs and how existing unit tests are organized. Done should include an `enable_logging` option defaulting to `false`, logging for models, and unit-test coverage for the validation behavior.

Written by the indexing model from the issue text.