protected List<? extends Path<?>> getKeys() {
        if (!keys.isEmpty()) {
            return keys;
        } else if (entity.getPrimaryKey() != null) {
            return entity.getPrimaryKey().getLocalColumns();
        } else {
            throw new IllegalStateException("No keys were defined, invoke keys(..) to add keys");
        }
    }
Add the given Expression at the given position as a query flag

@param position position
@param flag query flag
@return the current object
querydsl-sql/src/main/java/com/querydsl/sql/dml/SQLMergeClause.java