Prior articles about Hrorm gave an introduction and a first lesson. You may wish to take a look at those if you haven’t before diving into this article, which will explain how Hrorm (a concise, declarative ORM library for Java) handles relationships between entities.
SQL RDBMS platforms remain somewhat unfashionable. Some people seem to believe in a myth that they are not scalable enough. This may be true for some very large data sets and applications, but unless you work for a company with a name like Google or Amazon, your data is probably easily manageable by most popular databases.