You are viewing a plain text version of this content. The canonical link for it is here.
Posted to user@cayenne.apache.org by Aristedes Maniatis <ar...@maniatis.org> on 2015/04/15 11:20:04 UTC
lightweight object fetching
Cayenne has very nice functionality for paging large lists of objects, so that the list is mostly hollow objects which are retrieved in a lazy fashion as they are accessed.
This works well to fetch 100,000 contacts in a list and only draw the ones visible to the user as they scroll. What works less well is the common use case of a list view where you might only want to show several attributes of a large record.
So a list of contacts might show just:
* firstName
* lastName
* totalSales
To fetch this data requires a query on contact with a prefetch to invoices. Lots of data being loaded from two or more tables and potentially a bit slow.
Some options:
1. Use DataRows. This is simple, but you lose some of the nice modelling/entity features from Cayenne. You have to type all the columns yourself.
2. Create a view in the database with just three columns (plus a PK) and create a separate read-only Cayenne model which maps to that view. But now you are hardcoding your application to a specific database and changing the representation is hard.
I want something half way between the two. SQLtemplate to fetch only the columns or aggregates I need, but mapped to some sort of read-only lightweight Cayenne entity.
Has anyone tried something like this?
Ari
--
-------------------------->
Aristedes Maniatis
GPG fingerprint CBFB 84B4 738D 4E87 5E5C 5EFA EF6A 7D2E 3E49 102A
Re: lightweight object fetching
Posted by Andrus Adamchik <an...@objectstyle.org>.
Use EJBQLQuery. It supports fetching individual columns (and does not require SQL).
We are working to support this in ObjectSelect as well.
Andrus
> On Apr 15, 2015, at 12:20 PM, Aristedes Maniatis <ar...@maniatis.org> wrote:
>
> Cayenne has very nice functionality for paging large lists of objects, so that the list is mostly hollow objects which are retrieved in a lazy fashion as they are accessed.
>
> This works well to fetch 100,000 contacts in a list and only draw the ones visible to the user as they scroll. What works less well is the common use case of a list view where you might only want to show several attributes of a large record.
>
> So a list of contacts might show just:
>
> * firstName
> * lastName
> * totalSales
>
> To fetch this data requires a query on contact with a prefetch to invoices. Lots of data being loaded from two or more tables and potentially a bit slow.
>
> Some options:
>
> 1. Use DataRows. This is simple, but you lose some of the nice modelling/entity features from Cayenne. You have to type all the columns yourself.
>
> 2. Create a view in the database with just three columns (plus a PK) and create a separate read-only Cayenne model which maps to that view. But now you are hardcoding your application to a specific database and changing the representation is hard.
>
> I want something half way between the two. SQLtemplate to fetch only the columns or aggregates I need, but mapped to some sort of read-only lightweight Cayenne entity.
>
>
> Has anyone tried something like this?
>
>
> Ari
>
>
>
> --
> -------------------------->
> Aristedes Maniatis
> GPG fingerprint CBFB 84B4 738D 4E87 5E5C 5EFA EF6A 7D2E 3E49 102A
>
Re: lightweight object fetching
Posted by Andrus Adamchik <an...@objectstyle.org>.
Use EJBQLQuery. It is old style and requires String concatenation, but supports fetching individual columns and does not require SQL:
EJBQLQuery q =
new EJBQLQuery("SELECT p.firstName, p.lastName, sum(s.sales) FROM Person p INNER JOIN p.sale s");
We are working to support this in ObjectSelect as well.
Andrus
> On Apr 15, 2015, at 12:20 PM, Aristedes Maniatis <ar...@maniatis.org> wrote:
>
> Cayenne has very nice functionality for paging large lists of objects, so that the list is mostly hollow objects which are retrieved in a lazy fashion as they are accessed.
>
> This works well to fetch 100,000 contacts in a list and only draw the ones visible to the user as they scroll. What works less well is the common use case of a list view where you might only want to show several attributes of a large record.
>
> So a list of contacts might show just:
>
> * firstName
> * lastName
> * totalSales
>
> To fetch this data requires a query on contact with a prefetch to invoices. Lots of data being loaded from two or more tables and potentially a bit slow.
>
> Some options:
>
> 1. Use DataRows. This is simple, but you lose some of the nice modelling/entity features from Cayenne. You have to type all the columns yourself.
>
> 2. Create a view in the database with just three columns (plus a PK) and create a separate read-only Cayenne model which maps to that view. But now you are hardcoding your application to a specific database and changing the representation is hard.
>
> I want something half way between the two. SQLtemplate to fetch only the columns or aggregates I need, but mapped to some sort of read-only lightweight Cayenne entity.
>
>
> Has anyone tried something like this?
>
>
> Ari
>
>
>
> --
> -------------------------->
> Aristedes Maniatis
> GPG fingerprint CBFB 84B4 738D 4E87 5E5C 5EFA EF6A 7D2E 3E49 102A
>
Re: lightweight object fetching
Posted by do...@xsinet.co.za.
Hi Ari
Not sure if this is what you are looking for, but this is what I did once:
1. Create a dbEntity in the modeler that has all the columns you are
interested in from the various tables. (So this dbEntity is more like a view
as it doesn't represent a real table.)
2. Create a read only ObjEntity in the modeler based on the 'fake'
dbEntity.
3. Create a SQLTemplate query that will retrieve the data from the various
tables.
4. Perform the query as usual (no datarows)
For example I did something like:
SQLTemplate reportTemplateQry = new SQLTemplate
(
ReportTemplateLine.class,
"select A.NAME, B.ORDER from RPT_GROUPS as B, PRT_TEMPLATE as A "
+"where .... "
);
List<ReportTemplateLine> lines = ....performQuery( reportTemplateQry );
Regards
Jurgen
-----Original Message-----
From: Aristedes Maniatis
Sent: Wednesday, April 15, 2015 11:20 AM
To: user@cayenne.apache.org
Subject: lightweight object fetching
Cayenne has very nice functionality for paging large lists of objects, so
that the list is mostly hollow objects which are retrieved in a lazy fashion
as they are accessed.
This works well to fetch 100,000 contacts in a list and only draw the ones
visible to the user as they scroll. What works less well is the common use
case of a list view where you might only want to show several attributes of
a large record.
So a list of contacts might show just:
* firstName
* lastName
* totalSales
To fetch this data requires a query on contact with a prefetch to invoices.
Lots of data being loaded from two or more tables and potentially a bit
slow.
Some options:
1. Use DataRows. This is simple, but you lose some of the nice
modelling/entity features from Cayenne. You have to type all the columns
yourself.
2. Create a view in the database with just three columns (plus a PK) and
create a separate read-only Cayenne model which maps to that view. But now
you are hardcoding your application to a specific database and changing the
representation is hard.
I want something half way between the two. SQLtemplate to fetch only the
columns or aggregates I need, but mapped to some sort of read-only
lightweight Cayenne entity.
Has anyone tried something like this?
Ari
--
-------------------------->
Aristedes Maniatis
GPG fingerprint CBFB 84B4 738D 4E87 5E5C 5EFA EF6A 7D2E 3E49 102A
Re: lightweight object fetching
Posted by Michael Gentry <mg...@masslight.net>.
Hi Ari,
We chose option 2 and created a view with a read-only modeler mapping.
This was especially more useful for us since we had quite a few joins we
were flattening out for putting into an overview table and we kept all that
activity in the database instead of round-trips between our server and the
database.
I was going to mention creating "subset" entities, too, but Jurgen already
suggested that.
mrg
On Wed, Apr 15, 2015 at 5:20 AM, Aristedes Maniatis <ar...@maniatis.org>
wrote:
> Cayenne has very nice functionality for paging large lists of objects, so
> that the list is mostly hollow objects which are retrieved in a lazy
> fashion as they are accessed.
>
> This works well to fetch 100,000 contacts in a list and only draw the ones
> visible to the user as they scroll. What works less well is the common use
> case of a list view where you might only want to show several attributes of
> a large record.
>
> So a list of contacts might show just:
>
> * firstName
> * lastName
> * totalSales
>
> To fetch this data requires a query on contact with a prefetch to
> invoices. Lots of data being loaded from two or more tables and potentially
> a bit slow.
>
> Some options:
>
> 1. Use DataRows. This is simple, but you lose some of the nice
> modelling/entity features from Cayenne. You have to type all the columns
> yourself.
>
> 2. Create a view in the database with just three columns (plus a PK) and
> create a separate read-only Cayenne model which maps to that view. But now
> you are hardcoding your application to a specific database and changing the
> representation is hard.
>
> I want something half way between the two. SQLtemplate to fetch only the
> columns or aggregates I need, but mapped to some sort of read-only
> lightweight Cayenne entity.
>
>
> Has anyone tried something like this?
>
>
> Ari
>
>
>
> --
> -------------------------->
> Aristedes Maniatis
> GPG fingerprint CBFB 84B4 738D 4E87 5E5C 5EFA EF6A 7D2E 3E49 102A
>