Filter with Spring-Data and QueryDSL on nullable reference attribute

Question

I have following issue. I am using Jquery Datatable serverside and I am now implementing the search box. But I have an issue there in special case, when a dataset has an attribute, what is "null". So the dataset will not be found although it should found cos it matches on one attibute.

The situtation in beginning is like follows. You see there is a dataset with apprentice Fabio Bartels, who has not Fachrichtung. And a dataset with Viktoria.

Now when I search for Viktoria, the filter works as expected:

When I search for Fabio, then Dataset is not found:

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The problem I have is, that I don't know how to handle the filter, that a attribute will only be validated against the search string when the attribute is not null.

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Serverside Java Classes see like follows:

QueryClass:

class ContractSearchQuery {

private static QContract contract = QContract.contract;

static BooleanExpression getPredicate(final ContractSearch filter) {
    BooleanExpression predicate;

    if (filter == null || filter.isEmpty()) {
        // SHOW ALL PREDICATE ...
    } else {
        final String search = filter.getSearch();

        final List<BooleanExpression> expressions = new ArrayList<BooleanExpression>();

        // EXPRESSIONS CURRENTLY ONLY ON AUSZUBILDENDER AND FACHRICHTUNG
        // FOR SHOWCASE
        expressions.add(containsApprenticeName(search)); // AUSZUBILDENDER
        expressions.add(containsSpecialisation(search)); // FACHRICHTUNG

        BooleanExpression expression = expressions.get(INTEGER_ZERO);
        for (int i = 1; i < expressions.size(); i++) {
            expression = expression.or(expressions.get(i));
        }

        predicate = expression;
    }
    return predicate;
}

private static BooleanExpression containsApprenticeName(final String search) {
    final BooleanExpression expLastName = contract.apprentice.lastName.containsIgnoreCase(search);
    final BooleanExpression expFirstName = contract.apprentice.firstName.containsIgnoreCase(search);
    return expLastName.or(expFirstName);
}

private static BooleanExpression containsSpecialisation(final String search) {
    return contract.companyOccupationCombination.occupationCombination.specialisation.name.containsIgnoreCase(search);
}

}

Spring-Data-Repository Call:

final PageRequest pageRequest = new PageRequest(firstResult / maxResults, maxResults, orderSort);
final Page<Contract> page = contractRepository.findAll(predicate, pageRequest);
return page.getContent();

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Database:

By the way I recognized when I do direct request against my db with joining Specialisation Table, then I only get Fabio as record, when not joining Specialisation, I get all three persons. Maybe somethings to do with my issue:

select a.first_name, a.last_name from contract c 
    join company_occupation_combination coc on c.company_occupation_combination = coc.id 
    join occupation_combination oc on coc.occupation_combination = oc.id
    join apprentice a on c.apprentice = a.id

Result: "Fabio";"Bartels" "Viktoria";"Kruczek" "Lina";"Ehleiter"

With Join:

select a.first_name, a.last_name from contract c 
    join company_occupation_combination coc on         c.company_occupation_combination = coc.id 
    join occupation_combination oc on coc.occupation_combination = oc.id
    join specialisation s on oc.specialisation = s.id 
    join apprentice a on c.apprentice = a.id

Result: "Viktoria";"Kruczek"

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EDIT: Okay, on db site I found out (with Hibernate and JPA I start forgetting SQL-Basices ;-)), that I need a left join for the nullable relation, so my query should result to an sql like:

select a.first_name, a.last_name from contract c 
join company_occupation_combination coc on c.company_occupation_combination = coc.id 
join occupation_combination oc on coc.occupation_combination = oc.id
left join specialisation s on oc.specialisation = s.id 
join apprentice a on c.apprentice = a.id

====

So my question is, how can I manage left Join when I have a Query-Class using QueryDSL and Spring-Data-Repository like mentioned above?

Answer 1

I really like working with Spring-Data and Query-DSL, cos it makes my code really tidy. But I am really suprised, that for the case of nullable references there seems no solution. Sure you can use another solution like @vtorosyan mentioned and thank you again for that solution, but when you project is builded up with combination of QueryDSL and Spring-Data, you really don't want to bring a second style in your application.

But I needed a solution, so I did now the follows.

The point of the issue was, that when I used data from a nullable entity, a join has been executed what hided the datasets, who had a null reference on it, see examples above. What I now did and I hope I will not get another issue then with that solution on later time of that project. I did the null references to not null and defined something like null-record.

Example I added a record for specialisation like

ID NAME

0 Keine

Instead of null I now use that record what has until now following effects:

First my table shows now "Keine" (engl. "None") for all attributes what are not set. It looks more consistent when having a textoutput then empty string.

Now I can explicitly search for "Keine", when I am interested for data records what have no specialisation set.

And my searchbox works as expected for records, which have no speciafication set. (THAT WAS MY ISSUE FROM BEGINNING WHAT I WANTED TO SOLVE):

Additional to that searchbox I use a modal dialog for filtering. Now I can explicitly filter "Keine" for "nullable" records:

If you think there is another good solution for that issue without rebuild code using Spring-Data and QueryDSL konsequently, don't hesitate to post ;-)

Answer 2

If you really need left join, you can't achieve that via predicate (instead it is possible via sub-query)

To be able to do left-join, you will need JPAQuery. Assuming you have already configured repositories, and able to use EntitiManager, implement ContractRepositoryCustom , so that in your implementation you can have

@PersistenceContext(unitName = "unitname")
protected EntityManager entityManager;

public List<Contract> findAllContracts() {
    return new JPAQuery(entityManager, HQLTemplates.DEFAULT)
            .from(QContract.contract)
            .join(QContract.contract.companyOccupationCombination, QCompanyOccupationCombination.companyOccupationCombination)
            .join(QCompanyOccupationCombination.companyOccupationCombination.occupationCombination, QOccupationCombination.occupationCombination)
            .leftJoin(QOccupationCombination.occupationCombination.specialization, QSpecialization.specialization)
            .join(QSpecialization.specialization.apprentice, QApprentice.apprentice)
            .list(QContract.contract);
}

And for pagination you always apply limit(maxResults) and offset(firstResult)