Getting fewer columns with hibernate

Question

I have a table with 11 columns, but I need to get only 2 of them in my application, I'm using spring/hibernate/DAO combination. For now I have a domain class which includes all 11 fields, and mapping file which maps all 11 columns in table. How do I use get just 2 of them not all?

Answer 1

Try:

SELECT myEntity.one, myEntity.two FROM MyEntity myEntity

You can even do :

SELECT new MyEntityDescription(myEntity.one, myEntity.two) FROM MyEntity myEntity

to get a list of entity descriptions.

Answer 2

If you never need more than those 2 columns of the table, you could change your hibernate mapping to map only those 2 needed columns to the entity class. Only map those table columns you want to access in your application. Keep in mind, that database constraints on the "ignored" columns can be violated like not null constraints, foreign keys or unique constraints.

Answer 3

Either:

1. Use projections - Pro: nothing to add - Con: Not typesafe (the result is a List of rows where each row is anObject[]):

   select f.foo, f.bar from FatEntity f
   

2. Use a constructor expression in the SELECT clause (the specified class is not required to be an entity or to be mapped to the database) - Pro: typesafe solution - Con: More classes, unless you reuse FatEntity as holder in which case many fields will be null:

   select new com.acme.FatEntityDetails(f.id, f.foo, f.bar) from FatEntity f
   

   Note that if an entity class name is specified in the SELECT NEW clause, the resulting entity instances are in the new state (no persistent identity).

3. Use another entity mapped on the same table with only the required fields - Pro: It's a real entity that you can modify and update - Con: More classes.

   from LightEntity
   

The main differences between #2 and #3 are:

• #2 doesn't require the holder to be an entity at all.
• the holder in #2 could be an entity mapped on another table.
• if #2 returns entities, they are in a new state (this might be a problem, or not).