public Set<ContentPackInstallation> findByContentPackId(ModelId id) {
        final DBQuery.Query query = DBQuery.is(ContentPackInstallation.FIELD_CONTENT_PACK_ID, id);
        try (final DBCursor<ContentPackInstallation> installations = dbCollection.find(query)) {
            return ImmutableSet.copyOf((Iterator<ContentPackInstallation>) installations);
        }
    }
@Test
    @MongoDBFixtures("ContentPackInstallationPersistenceServiceTest.json")
    public void findByContentPackIdWithInvalidId() {
        final Set<ContentPackInstallation> contentPacks = persistenceService.findByContentPackId(ModelId.of("does-not-exist"));

        assertThat(contentPacks).isEmpty();
    }