code: private int getUriid(String uri) {
  		Integer id = uriRefIds.getId(uri);
  		if (id != null)
  			return id;
  		return -2;
  	}
docs: We look for the ID in the list of IDs, if it's not there, then we return -2,
  which we know will never appear on the DB. This is correct because if a
  constant appears in a query, and that constant was never inserted in the
  DB, the query must be empty (that atom), by putting -2 as id, we will
  enforce that.
  
  @param uri
  @return