How to execute a JPAQuery with pagination using Spring Data and QueryDSL - spring

Question

I have this request working good with queryDSL :
Iterable<AO> query_result = new JPAQuery(entityManager).from(ao)
.leftJoin( ao.lots , lot )
.leftJoin( ao.acs , ac )
.where(where).distinct()
.list(ao);
But what is its equivalent if we use it with spring data jpa
ao_respository.findAll(Predicate arg0, Pageable arg1);
Because i want to return a Page and just with querydsl it doesn't implement Page without spring data jpa.
I try to put my where in Predicate arg0 but i got this exception
Undeclared path 'lot '. Add this path as a source to the query to be able to reference it
where lot is declared as QLot lot = QLot.lot;

Answer 1

I created my own Page class and executed the query like this:
JPAQuery query = new JPAQuery(entityManager).from(ao)
.leftJoin( .. ).fetch()
.leftJoin( .. ).fetch()
...
.where(where)
MaPage<AO> page = new MaPage<AO>();
page.number = pageNumber+1;
page.content = query.offset(pageNumber*pageSize).limit(pageSize).list(ao);
page.totalResult = query.count();
My Page class:
public class MaPage<T> {
public List<T> content;
public int number;
public Long totalResult;
public Long totalPages;
...
}
It works but I got this warning
nov. 21, 2014 6:48:54 AM
org.hibernate.hql.internal.ast.QueryTranslatorImpl list WARN:
HHH000104: firstResult/maxResults specified with collection fetch;
applying in memory!

Answer 2

Returning a Page:
JPAQuery query =
...
.orderBy(getOrderSpecifiers(pageable, MyEntity.class))
.limit(pageable.getPageSize())
.offset(pageable.getOffset());
long total = query.fetchCount();
List<MyEntity> content = query.fetch();
return new PageImpl<>(content, pageable, total);
And I created this function to get OrderSpecifier:
private OrderSpecifier[] getOrderSpecifiers(#NotNull Pageable pageable, #NotNull Class klass) {
// orderVariable must match the variable of FROM
String className = klass.getSimpleName();
final String orderVariable = String.valueOf(Character.toLowerCase(className.charAt(0))).concat(className.substring(1));
return pageable.getSort().stream()
.map(order -> new OrderSpecifier(
Order.valueOf(order.getDirection().toString()),
new PathBuilder(klass, orderVariable).get(order.getProperty()))
)
.toArray(OrderSpecifier[]::new);
}

Answer 3

If you have a working, complex query in querydsl and you want to use springdata pagination, you have to:
make your querydsl/repository method return Page<T>
Page<YourEntity> yourSelect(Pageable aPageable)
use querydsl offset and limit to page your result set
List<YourEntity> theResultList = jpaQueryFactory
.select(<whatever complext jpaquery you like>)
.offset(aPageable.getOffset())
.limit(aPageable.getPageSize())
.fetch();
provide a LongSuplier counting all available results with respect to your query and use PageableExecutionUtils to return the result as Page
final long theCount = jpaQueryFactory
.selectFrom(<your select to count all results>)
.fetchCount();
return PageableExecutionUtils.getPage(theResultList, aPageable, () -> theCount);