code: protected Document createDocument(String spaceId, String key) {
          Document doc = new Document();
          doc.add(new StringField(FIELD_SPACE_ID, spaceId.trim(), Store.YES));
          doc.add(new StringField(FIELD_KEY, key.trim(), Store.YES));
          doc.add(new StringField(FIELD_ID, spaceId.trim() + ":" + key.trim(), Store.NO));
          return doc;
      }
docs: Create a {@link Document}, pre-filled with space-id and key fields.
  
  @param spaceId
  @param key
  @return