code: public <T, K> JacksonDBCollection<T, K> getReferenceCollection(JacksonCollectionKey collectionKey) {
          JacksonDBCollection<T, K> collection = referencedCollectionCache.get(collectionKey);
          if (collection == null) {
              collection = new JacksonDBCollection<T, K>(getDB().getCollection(collectionKey.getName()),
                      collectionKey.getType(), collectionKey.getKeyType(), objectMapper, features);
              referencedCollectionCache.put(collectionKey, collection);
          }
          return collection;
      }
docs: Get a collection for loading a reference of the given type
  
  @param collectionKey The key for the collection
  @return The collection