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From Craig L Russell <Craig.Russ...@Sun.COM>
Subject Behavior of projection queries (long)
Date Fri, 09 Jun 2006 01:11:02 GMT
Javadogs,

For the JDO maintenance release, we have to decide what the behavior  
is for projection queries of various kinds of fields. [There are no  
TCK tests for this behavior.] Consider this class:

class Company {
int id;
String symbol;
Date incorporated;
BigDecimal revenues;
Set<Department> departments;
Locale hqLocale;
Integer empCount;
Address hqAddress;
}

and these JDOQL queries:

Collection<DTOCompany> dtos = execute this query: "SELECT new  
DTOCompany(symbol, incorporated, revenues, departments, hqLocale,  
empCount, hqAddress) FROM Company ORDER BY id"

Collection<Company> cos = execute this query: "SELECT FROM Company  
ORDER BY id"

1. Now, iterate the collections of dtos and cos. For each  
corresponding result dto and co in the collection of results:

For each of symbol, incorporated, revenues, departments, hqLocale,  
empCount, and hqAddress:

Does dto.symbol == co.symbol?
Does dto.symbol.equals(co.symbol)?

2. Now, suppose I change the mutable second class fields of the  
instances I get from cos (the persistent instances).

tx.begin();
cos.incorporated.setTime(new Date().getMillis());
tx.commit();

tx.begin();
cos.departments.add(new Department());
tx.commit();

tx.begin();
cos.hqAddress.setState("CA");
tx.commit();

Have I modified the company instance in the database in any of these  
cases?

3.Now, suppose I change the mutable second class fields of the  
instances I get from dtos (the holders for the field values of the  
instances).

tx.begin();
dtos.incorporated.setTime(new Date().getMillis());
tx.commit();

tx.begin();
dtos.departments.add(new Department());
tx.commit();

tx.begin();
dtos.hqAddress.setState("CA");
tx.commit();

Have I modified the company instance in the database in any of the  
cases?

Here's a proposal to clarify the specification.

If mutable second class fields are selected by a projection query,  
the instances returned by the query are the instances of the owning  
first class instance. Changes made to the results are reflected in  
the database at transaction commit. There is no requirement that  
immutable fields returned by a projection query are identical to  
corresponding fields in cached instances.

The implication is that a projection query that selects mutable  
second class fields has the effect of instantiating the owning first  
class instance and associating the mutable second class fields in the  
query result with the owning first class instance. So selecting first  
name, last name, age, and ssn returns only values not associated with  
persistent instances. But selecting incorporated, departments, or  
hqAddress will instantiate the first class instances and its default  
fetch group.

  Craig

Craig Russell
Architect, Sun Java Enterprise System http://java.sun.com/products/jdo
408 276-5638 mailto:Craig.Russell@sun.com
P.S. A good JDO? O, Gasp!


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