* The value is, e.g. _id <br>
     * comment: Field name for internal ID in the index.
     * @return The value of found property. (NotNull: if not found, exception but basically no way)
     */
    String getIndexFieldId();

    /**
     * Get the value for the key 'index.field.version'. <br>
     * The value is, e.g. _version <br>
     * comment: Field name for version in the index.