Hi Tom,
I tested with TopLink-Essentials, which is the default JPA persistence
provider bundled with Glassfish. TopLink gets the results of the Empoyee
queries from the cache.
Caching behavior is not prescribed by the spec. It highly depends on the
persistence provider, how many queries will be executed in your situation.
-- markus.
Tom Mutdosch wrote:
> Hi Markus,
>
> Thanks for the reply. I know there's faster ways to get the data that
> I need, but if I did do it this way (n+1 queries) do you know if/how
> JPA caches this data? Once I queried for all of the Departments
> (containing their Employees), when I executed a separate query to get
> the filtered set of Employees for each Department object, would it go
> to the Database each time? Or would it just retrieve the Employees
> from a cache somehow? I couldn't find anything on this when I
> searched the spec.
>
> Thanks for the helpful info,
> Tom
>
>
>
> Markus Fuchs wrote:
> > Hi Tom,
> >
> > Tom Mutdosch wrote:
> > > Hi Markus,
> > >
> > > Thanks for the reply. That makes sense; I just wasn't sure if
> JPA was
> > > able to more easily handle these more complex scenarios. I'll give
> > > you an example of where I would want to do this:
> > >
> > > I have a JSF page with a datatable showing a list of departments -
> > > each department row shows relevant fields (department name, ID, etc)
> > > as well as a nested list of Employees with 15 years of service. It
> > > would be nice if I could just bind the result of a single JPA
> query to
> > > that datatable. That is, I could just bind the list of Departments,
> > > and each one would have the appropriate list of Employees I want to
> > > display.
> > >
> > > So in JPA to do this, I have to do n + 1 separate queries? That is,
> > > one query to retrieve the departments that I want to display, and
> then
> > > for each department another query to get the filtered list of its
> > > Employees?
> > >
> > That sounds correct.
> > > That is doable, but it does complicate things a bit in that in my
> JSF
> > > page I can't just bind a list of Departments to my datatable; I
> would
> > > need to bind a list of DepartmentBeans that would wrapper my
> > > Department and for the employees property would instead return the
> > > results of a separate query to populate the Employees list for each
> > > row. Does that sound correct?
> > >
> > I'm not an JSF expert, but you'd need to execute n + 1 queries.
> >
> > Regards,
> >
> > -- markus.
> > > Thanks
> > > Tom
> > >
> > > Markus Fuchs wrote:
> > >> Hi Tom,
> > >>
> > >> JPA queries are not intended to return incompletely initialized
> fields.
> > >> As the Department in question really contains Employees besides
> those
> > >> having more than 15 years of service, the employeeCollection will
> > always
> > >> contain all Employees from that Department.
> > >>
> > >> A query returning a Collection containing the Employees with
> more that
> > >> 15 years of service is:
> > >>
> > >> select e from Employee e where e.department.deptno = 100 and
> > >> e.yearsOfService >= 15
> > >>
> > >> Tom Mutdosch wrote:
> > >> > Hi there,
> > >> > I have a question regarding JPA Query
> > >> >
> > >> > I have a DEPARTMENT entity with a one-to-many relationship to an
> > >> > EMPLOYEE entity. Say I want to get "departments with a
> department
> > >> > number of 100 and containing EMPLOYEES with more than 15
> years of
> > >> > service".
> > >> >
> > >> > Is it possible to get back a single result containing the
> DEPARTMENT
> > >> > object; and its relationship collection list consisting of
> only the
> > >> > EMPLOYEES matching the criteria?
> > >> >
> > >> No. Please see above.
> > >>
> > >> > I've tried creating various queries using joins, but they always
> > >> > return me back the DEPARTMENT I want, but the its employee list
> > >> > contains ALL of the employees belonging to that department,
> not just
> > >> > the ones matching the "15 years of service" filter.
> > >> >
> > >> > For example:
> > >> > select d from Department d LEFT JOIN d.employeeCollection e
> WHERE
> > >> > d.deptno = 100 AND e.yearsOfService >= 15
> > >> >
> > >> > This returns one result - a Department with ALL of the employees
> > that
> > >> > belong to that department (not just the ones matching
> > >> e.yearsOfService
> > >> > >= 15)
> > >> >
> > >> > I also tried this:
> > >> > select d, e from Department d LEFT JOIN d.employeeCollection
> e WHERE
> > >> > d.deptno = 100 AND e.yearsOfService >= 15
> > >> >
> > >> > And that returns 3 results, each an array containing a
> Department
> > and
> > >> > Employee. This is giving me the right data (there are 3
> employees
> > >> > that match the criteria), but I really just want to get back a
> > single
> > >> > department result containing the filtered employees. Is this
> > >> possible?
> > >> >
> > >> No. How should the expected result look like in this case? An array
> > >> containing one Department and three Employees?
> > >>
> > >> HTH,
> > >>
> > >> -- markus.
> >
>
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