在HQL中指定字段列表似乎不起作用

Question

I have the following HQL in Hibernate using Spring MVC.

List<Colour>list=session.createQuery("from Colour order by colourId desc")
.setFirstResult((currentPage-1)*rowsPerPage)
.setMaxResults(rowsPerPage).list();

It works and returns a list of rows from the colour table (actually operates upon the Colour entity (POJO) that I can understand) in Oracle 10g.

What if I need to retrieve a list fields, I'm trying the following.

List<Colour>list=session.createQuery("colourId, colourName, colourHex from Colour order by colourId desc")
.setFirstResult((currentPage-1)*rowsPerPage)
.setMaxResults(rowsPerPage).list();

It ends with an excpetion

java.lang.IllegalArgumentException: node to traverse cannot be null!

In some articles, it was mentioned that the following version of HQL should (or may) work

List<Colour>list=session.createQuery("select colourId, colourName, colourHex from Colour order by colourId desc")
.setFirstResult((currentPage-1)*rowsPerPage)
.setMaxResults(rowsPerPage).list();

but unfortunately, it also didn't work for me. Using the createSQLQuery() method to execute native SQL would work but I want to stick to the createQuery() method with HQL unless it's absolutely necessary. How can I specify a list of fields in HQL?

Answer 1

I agree with yorkw's comment. If you select properties in your query then you cannot ask for a List<Colour> object to be returned from a call to .list() .

Instead you should do this

    List<Object[]> rows = session.createQuery("select c.colourId, c.colourName, c.colourHex " +
        " from Colour c " + 
        " order by c.colourId desc").list();

Then iterate over the list object and instantiate your objects. Or whatever you need to do.

    for ( Object[] row : rows ) {
        Long colourId = (Long)row[0];
        // ... etc
    }

Answer 2

Why don't you try creating a map? Something like this:

SELECT NEW MAP( colour.colourId AS id
              , colour.colourName AS name ...) 
  FROM Colour colour 
  ORDER BY colour.colourId

I use the alias for Colour "colour" so hibernate knows from which entity is the property I am referencing, I am implying all those properties are from the same entity, if not, then check your referencing!