JPA: DELETE WHERE does not delete children and throws an exception

DELETE (and INSERT) do not cascade via relationships in JPQL query. This is clearly spelled in specification:

A delete operation only applies to entities of the specified class and
its subclasses. It does not cascade to related entities.

Luckily persist and removal via entity manager do (when there is cascade attribute defined).

What you can do:

• fetch all Mother entity instances that should be removed.
• for each of them call EntityManager.remove().

Code is something like this:

String selectQuery = "SELECT m FROM Mother m WHERE some_condition";  
List<Mother> mothersToRemove = entityManager
    .createQuery(selectQuery)
    .getResultStream()
    .forEach(em::remove);