Sorry, the query element is

QueryBuilder qb = fts.getSearchFactory().buildQueryBuilder()
.forEntity(WallPost.class).get();
[...]
qb.keyword().onField("sponsor.profile.tag.id").matching(trc.getTag().getId()).createQuery()
[...]
FullTextQuery ftq = fts
.createFullTextQuery(
mj.createQuery(),
WallPost.class)
.setProjection(ProjectionConstants.ID)
.setResultTransformer(new ProjectionFirstResultTransformer())
.setFirstResult(start)
.setMaxResults(max)
;