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From: Andy Jefferson <andy@jpox.org>
To: jdo-dev@db.apache.org
Subject: Re: Embedded collections of non-PC objects
Date: Thu, 14 Jul 2005 20:22:50 +0100
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Hi Craig,

> Is it possible to default the value of the <field> serialized
> attribute to "true" for embedded non-PC types? That would immediately
> solve our current Collection, Set, and Map issues, without having
> extra stuff in the metadata file.

Anything is possible ...

> The .jdo file specifies the embedded-element, embedded-key, and
> embedded-value attributes for the relevant fields, so simply changing
> the defaults would make it all work.
>
> I looked at the spec again and it doesn't look like the serialized
> attribute should be required, so it would be better if it were
> defaulted
> <spec>
> The embedded attribute specifies whether the field should be stored
> as part of the containing instance instead of as its own instance in
> the datastore. It must be specified or default to "true" for fields
> of primitive types, wrappers, java.lang, java.math, java.util,
> collection, map, and array types specified above; and "false"
> otherwise. While a compliant implementation is permitted to support
> these types as first class instances in the datastore, the semantics
> of embedded=3D=E2=80=9Dtrue=E2=80=9D imply containment. That is, the embe=
dded instances
> have no independent existence in the datastore and have no Extent
> representation.
> The semantics of embedded applied to collection, map, and array types
> applies to the structure of the type, not to the elements, keys, and
> values. That is, the collection itself is considered separate from
> its contents. These may separately be specified to be embedded or
> not, using embedded-element, embedded-key, and embedded-value
> attributes of the collection, array, and map elements.
> The embedded attribute applied to a field of a persistence-capable
> type is a hint to the implementation to treat the field as if it were
> a Second Class Object. But this behavior is not further specified and
> is not portable.
> </spec>

Well this does beg the question, where is the difference between "serialize=
d"=20
and "embedded" at <field> level then ?
If I specify <field embedded=3D"true">  for a collection, is this the same =
as=20
<field serialized=3D"true"> for the collection ?  "serialized" doesn't have=
 a=20
description in 18.13 btw.

Personally I would expect a Collection<String> or Map<String, String> to=20
default to having the elements queryable, and embedding them (by default) i=
s=20
not going to give me that. Perhaps I just disagree with the spec on this=20
issue :-)

Perhaps some examples would be beneficial ? (in the spec). Not that I'm try=
ing=20
to increase your workload :-0. I see one example of "embedded" and that is=
=20
<embedded> defining fine details of how to embed an object (which is what I=
=20
would expect to use this keyword for anyway), and which seems a long way fr=
om=20
what we're talking about here. If I'm not understanding this then you can b=
et=20
SUN's vast bank balance that users will have a hard time working it out, an=
d=20
I'm not going to spend my spare time explaining it to them


=2D-=20
Andy
Java Persistent Objects - JPOX