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From: Andrus Adamchik <andrus@objectstyle.org>
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Subject: Re: ColumnSelect API + Persistent objects
Date: Mon, 13 Mar 2017 11:26:53 +0300
References: <CAMi+qzBJayK3Xk36H6_Sxj+AsiReM_FMi_UPnkvE8SwL00dHMw@mail.gmail.com>
 <a1db1e09-5c4f-72f2-376b-ba8b49c25f29@maniatis.org>
 <CAMi+qzDX5hMc_1fK0X6tF9qZcmf4q7EZOKuZ6Tw1Zt8SfZSJxg@mail.gmail.com>
 <BA2CB857-0DD1-4309-8D95-238E4D61DF23@objectstyle.org>
 <A66394EA-ABB6-4202-8521-E4B19A12C99D@objectstyle.org>
 <9978ec72-3033-9675-4434-21c28e6bc5b3@maniatis.org>
To: dev@cayenne.apache.org
In-Reply-To: <9978ec72-3033-9675-4434-21c28e6bc5b3@maniatis.org>
Message-Id: <8EAE441C-5264-49DF-BFA1-07AACFECF709@objectstyle.org>
archived-at: Mon, 13 Mar 2017 08:27:03 -0000

> Oh, I get it now. You are suggesting in option (2) that Cayenne =
creates a CROSS JOIN in sql.

Actually not a CROSS join. It will be a regular INNER JOIN based on =
relationship from the query root. JPA spec defines CROSS join =
operations. We don't (yet?).

> Sometimes you want to fetch specific columns as read-only for =
performance reasons. I get that. You save on fetching data and you save =
time on constructing full ObjEntities.


Yep. A typical use case for intermixing DataObjects and scalars in the =
result is this: [<artist>, <count_of_paintings>] that gives us each =
object's to-many counts without resolving to-many relationships. What I =
am unsure though is whether [<artist_name>, <painting>] result is of any =
use to anyone.

> Are users going to be confused that the ObjEntities are bound to a =
context and editable/committable, but the raw columns are not?

I am not concerned about this. I think everyone should understand that =
only DataObjects can be committed. Any other representations is =
non-updateable data.

Andrus


> On Mar 13, 2017, at 10:29 AM, Aristedes Maniatis <ari@maniatis.org> =
wrote:
>=20
> On 13/3/17 5:57pm, Andrus Adamchik wrote:
>>=20
>>>>>=20
>>>>> List<Object[]> result =3D ObjectSelect.query(Artist.class)
>>>>>      .columns(Artist.ARTIST_NAME, Artist.PAINTING_ARRAY)
>>>>>      .select(context);
>>=20
>>=20
>>>> For me it's not clear when do we need direct List result for toMany
>>>> relationship instead of using Prefetch (or even selecting related
>>>> entities explicitly).
>>=20
>> Let's ponder on this some more. In the example above there are two =
possible result mappings of to-many - each cell at index 1 can store =
either (1) a List<Painting> or (2) a Painting. Of course in the second =
case Artist name at index 0 will be repeated for each painting of a =
given Artist (so the result is flattened).=20
>>=20
>> In the proposed solution we throw in the first case and support the =
second. I'd like to hear opinions on the usefulness of either of the =
cases, and maybe some real life examples.
>=20
>=20
> Oh, I get it now. You are suggesting in option (2) that Cayenne =
creates a CROSS JOIN in sql.
>=20
> Surely most of the time we are steering users to creating ObjEntities =
where possible; then they have the full access to follow relations, =
prefetching, pagination, etc, etc.
>=20
> Sometimes you want to fetch specific columns as read-only for =
performance reasons. I get that. You save on fetching data and you save =
time on constructing full ObjEntities. But then why would you want to =
embed full ObjEntities in the result? Any performance benefits are =
surely out the window. So what's the point and the use-case?
>=20
>=20
>=20
> Are users going to be confused that the ObjEntities are bound to a =
context and editable/committable, but the raw columns are not?
>=20
> result.get(0)[3][1].setName("The scream")
> result.get(0)[2] =3D "Munch"
> context.commit()
>=20
>=20
>=20
> What is the output of this:
>=20
> result =3D ObjectSelect.query(Artist.class)
>       .columns(createSelf(Artist.class))
>       .select(context);
>=20
> Is that the same as=20
>=20
> result =3D ObjectSelect.query(Artist.class)
>       .select(context);
>=20
> would the generated SQL be the same?
>=20
> Ari
>=20
>=20
>=20
>=20
> --=20
> -------------------------->
> Aristedes Maniatis
> GPG fingerprint CBFB 84B4 738D 4E87 5E5C  5EFA EF6A 7D2E 3E49 102A