RespositoryRestController.java

 Handler<Object, ResponseEntity<?>> entityHandler = new Handler<Object, ResponseEntity<?>>() {
      @Override public ResponseEntity<?> handle(Object linkedEntity) {

        if(attrMeta.isCollectionLike()) {
          Collection c = new ArrayList();
          Collection current = attrMeta.asCollection(entity);
          if(request.getMethod() == HttpMethod.POST && null != current) {
            c.addAll(current);
          }
          c.add(linkedEntity);
          attrMeta.set(c, entity);
        } else if(attrMeta.isSetLike()) {
          Set s = new HashSet();
          Set current = attrMeta.asSet(entity);
          if(request.getMethod() == HttpMethod.POST && null != current) {
            s.addAll(current);
          }
          s.add(linkedEntity);
          attrMeta.set(s, entity);
        } else if(attrMeta.isMapLike()) {
          Map m = new HashMap();
          Map current = attrMeta.asMap(entity);
          if(request.getMethod() == HttpMethod.POST && null != current) {
            m.putAll(current);
          }
          String key = rel.get();
          if(null == key) {
            throw new IllegalArgumentException("Map key cannot be null (usually the 'rel' value of a JSON object).");
          }
          m.put(rel.get(), linkedEntity);
          attrMeta.set(m, entity);
        } else {
          // Don't support POST when it's a single value
          if(request.getMethod() == HttpMethod.POST) {
            try {
              return negotiateResponse(request, HttpStatus.METHOD_NOT_ALLOWED, new HttpHeaders(), null);
            } catch(IOException e) {
              throw new IllegalStateException(e.getMessage(), e);
            }
          }
          attrMeta.set(linkedEntity, entity);
        }

        return null;
      }
    };