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Date: Fri,  9 Jun 2006 00:11:41 -0700
From: Erik Bengtson <erik@jpox.org>
To: JDO Expert Group <jdo-experts-ext@Sun.COM>,
 Apache JDO project <jdo-dev@db.apache.org>
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 <E16EB59B8AEDF445B644617E3C1B3C9C08373E@repbex01.amer.bea.com>
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Subject: RE: Behavior of projection queries (long)
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More inline

Quoting David Ezzio <dezzio@bea.com>:

> Hi Craig,
>
> Comments inline.
>
> David
>
>
>
> -----Original Message-----
> From: Craig L Russell [mailto:Craig.Russell@Sun.COM]
> Sent: Thu 6/8/2006 9:11 PM
> To: JDO Expert Group; Apache JDO project
> Subject: Behavior of projection queries (long)
>
> 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"
>

This query will raise JDOUserException, Collection fields cannot be projected.

> 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?
>
> ***
> NO
>

I'm not sure if would be a good idea to take the SCO instance from the FCO or
not. Maybe in pratical terms for users NOT, but logically YES.

> Does dto.symbol.equals(co.symbol)?
>
> ***
> YES
>
> 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?
>
> ***
> YES
>
>
> 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();
>
> ***
> Maybe, but only if Address is a first class object.
>
> 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.
>
> ***
> I'm sure you have a very good reason in mind.  Perhaps I missed it.
>

I see Craig's point, however as you point below it may be trick for users.

Collections and maps fields cannot be projected, so we have only non set fields
left and allowing them to affect the database would not be much benefit.

> One the one side, I fail to see the virtues of this proposal.  On the other
> side, the whole point of projections is to avoid instantiating first class
> objects (although the collections produced may be collections of first class
> objects).  Likewise, projected values should never be owned by first class
> objects.
>
> The proposal adds yet another corner case that will confuse, snare, and
> flabbergast application developers.
>
> In my opinion, the rule should always be: projected SCOs are never owned,
> projected FCOs are always managed.  Modifying unowned SCOs never has an
> effect on the database.  Modifying FCOs no matter how you get them always has
> an effect if the tx commits (except for the well know corner case of
> modifying just the unowned side of a bidirectional relationship).  That's
> what most developers would expect.
> ***
>
>   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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