Invalid slot switch attempt doesn't unslot

src/main/java/de/webalf/slotbot/model/Slot.java

@@ -123,25 +123,31 @@ public void slot(User user) {
 	}
 
 	/**
-	 * @see Slot#slot(User)
-	 * Doesn't trigger the slotUpdate
-	 *
-	 * @throws BusinessRuntimeException if the user is already slotted on this slot or the slot is already occupied
+	 * @throws BusinessRuntimeException if the given user can't be slotted to the slot
 	 */
-	void slotWithoutUpdate(@NonNull User user) {
+	public void assertSlotIsPossible(@NonNull User user) {
 		if (isSlotWithSlottedUser(user)) {
 			throw BusinessRuntimeException.builder().title("Die Person ist bereits auf diesem Slot").build();
 		} else if (!isEmpty()) {
 			throw BusinessRuntimeException.builder().title("Auf dem Slot befindet sich eine andere Person").build();
 		} else if (getEffectiveReservedFor() != null && user.getGuilds().stream().noneMatch(guild -> guild.equals(getEffectiveReservedFor()))) {
 			throw BusinessRuntimeException.builder().title("Dieser Slot ist für Mitglieder einer anderen Gruppe reserviert").build();
-		} else {
-			//Remove the user from any other slot in the Event
-			getEvent().findSlotOfUser(user).ifPresent(slot -> slot.unslotWithoutUpdate(user));
-			setUser(user);
 		}
 	}
 
+	/**
+	 * @see Slot#slot(User)
+	 * Doesn't trigger the slotUpdate
+	 *
+	 * @throws BusinessRuntimeException if the user is already slotted on this slot or the slot is already occupied
+	 */
+	void slotWithoutUpdate(@NonNull User user) {
+		assertSlotIsPossible(user);
+		//Remove the user from any other slot in the Event
+		getEvent().findSlotOfUser(user).ifPresent(slot -> slot.unslotWithoutUpdate(user));
+		setUser(user);
+	}
+
 	/**
 	 * Removes the given user from the slot if no other user occupies the slot
 	 *

src/main/java/de/webalf/slotbot/service/EventService.java

@@ -296,6 +296,8 @@ public void deleteEvent(Event event) {
 	public Event slot(@NonNull Event event, int slotNumber, UserDto userDto) {
 		Slot slot = event.findSlot(slotNumber).orElseThrow(ResourceNotFoundException::new);
 		User user = userService.find(userDto);
+
+		slot.assertSlotIsPossible(user);
 		event.unslotIfAlreadySlotted(user);
 		eventRepository.saveAndFlush(event);
 		slotService.slot(slot, user);