[fix] Restrict eager loading to @OnetoOne and @onetomany relationships (darrachequesne#39)

Previously, a JOIN FETCH was also added when dealing with a relationship that may return multiple lines (@onetomany, for example). Since Hibernate doesn't know how many lines it will need to construct your entities, it fetches a lot of lines and store them in memory, resulting in a big performance hit. In that case, the following warning is displayed: "HHH000104: firstResult/maxResults specified with collection fetch; applying in memory!". That commit thus restricts JOIN FETCH to @OnetoOne and @manytoone relationships.

Author: darrachequesne

src/main/java/org/springframework/data/jpa/datatables/repository/SpecificationFactory.java

@@ -111,8 +111,10 @@ public Predicate toPredicate(Root<T> root, CriteriaQuery<?> query, CriteriaBuild
  continue;
  }
  String[] values = column.getData().split(ESCAPED_ATTRIBUTE_SEPARATOR);
- if (root.getModel().getAttribute(values[0])
- .getPersistentAttributeType() == PersistentAttributeType.EMBEDDED) {
+ PersistentAttributeType type =
+ root.getModel().getAttribute(values[0]).getPersistentAttributeType();
+ if (type != PersistentAttributeType.ONE_TO_ONE
+ && type != PersistentAttributeType.MANY_TO_ONE) {
  continue;
  }
  Fetch<?, ?> fetch = null;