- provide an new configuration parameter in the administration.
- when the option is activated display a message (in real time) stating that the field is mandatory when it is empty. The save buttons must be disabled in that case.
I think the cost of making the feature work with JS deactivated is overkill (new validate action) for this use case. In the long run we'll need a generic validation mechanism on document save, something that would replace the current object validation mechanism (not completely implemented) but generic enough to allow validation fields of the document itself.