code: public static Specification<JpaTarget> hasNoActionInRolloutGroup(final Long group) {
          return (targetRoot, query, cb) -> {
              final ListJoin<JpaTarget, RolloutTargetGroup> rolloutTargetJoin = targetRoot
                      .join(JpaTarget_.rolloutTargetGroup, JoinType.INNER);
              rolloutTargetJoin.on(
                      cb.equal(rolloutTargetJoin.get(RolloutTargetGroup_.rolloutGroup).get(JpaRolloutGroup_.id), group));
  
              final ListJoin<JpaTarget, JpaAction> actionsJoin = targetRoot.join(JpaTarget_.actions, JoinType.LEFT);
              actionsJoin.on(cb.equal(actionsJoin.get(JpaAction_.rolloutGroup).get(JpaRolloutGroup_.id), group));
  
              return cb.isNull(actionsJoin.get(JpaAction_.id));
          };
      }
docs: {@link Specification} for retrieving {@link Target}s that have no Action
  of the {@link RolloutGroup}.
  
  @param group
  the {@link RolloutGroup}
  @return the {@link Target} {@link Specification}