Custom validation example

Hi,

I'm a bit confused on how to implement the following 'business rule':

What:
An assessment cannot be saved if there is already an assessment for a certain date, applicant and assessment-type. (assessment, applicant, assessment date are 'unique'). We might in the future have to add 'for a certain user domain/organization' ... so we don't think of solving this with an unique index on db level.

How:
Validation rule. With other words, find async the assessment with the same date, applicant and assessment-type as the assessment to be saved and returns 'all good' if none found.

With what:
Here it becomes a bit blurry. It seems the Verifier route is the way to go. But I get stuck Finding the assessments async.
And I don't really get if the EntityServerSaveInterceptor is the place to do this check.

Any help would be appreciated!