Components for building queries, executing them and processing results.
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README.md

AIS.PL Commons - Query

This library provides few abstractions usable while querying the data, like Selection and SearchResults, along with their Querydsl based implementation.

Usage examples

We will use simple domain model for the examples below

![Course - Participant diagram](src/site/resources/Course - Participant.png?raw=true "Course - Participant")

Reading all courses into list

Having JPAQuery you may create new Results instance with forQuery method, and request query results transforming into number of records, single result, List, or SearchResults using one of the Transformers.

import static pl.ais.commons.query.dsl.Transformers.list;
import static pl.ais.example.webapp.domain.model.QCourse.course;
...
final JPAQuery query = new JPAQuery(entityManager, jpqlTemplates).from(course);
final List<Course> courses = Results.forQuery(query).as(list(course));

The above code leads to following JPQL query:

select course from Course course

Counting all courses

Simple example, counting all records returned by the query.

import static pl.ais.commons.query.dsl.Transformers.numberOfResults;
import static pl.ais.example.webapp.domain.model.QCourse.course;
...
final JPAQuery query = new JPAQuery(entityManager, jpqlTemplates).from(course);
final Long coursesNo = Results.forQuery(query).as(numberOfResults());

The above code leads to following JPQL query:

select count(course) from Course course

Fetching one of the properties for course matching specified criteria

A little more complicated use case, where you specify criteria to be matched by returned results (using matching method), and fetch only single Course property (name) instead of the whole Course entity.

import static pl.ais.commons.query.dsl.Transformers.singleResult;
import static pl.ais.example.webapp.domain.model.QCourse.course;
...
final JPAQuery query = new JPAQuery(entityManager, jpqlTemplates).from(course);
final String courseName = Results.forQuery(query).matching(course.id.eq(1)).as(singleResult(course.name));

The above code leads to following JPQL query:

select course.name from Course course where course.id = 1

Fetching participant names for courses having more than 5 participants, along with the total number of matching participants.

Again we specify additional criteria to be matched by returned results, but this time we request to transform them into SearchResults instance. SearchResults is a container for holding query results, along with total number of records matching given criteria. It is very useful, when you want to paginate results using Selection (see examples below), in this case you fetch only subset of all records matching criteria, and are aware about the total number of matching records too.

import static pl.ais.commons.query.dsl.Transformers.searchResults;
import static pl.ais.example.webapp.domain.model.QCourse.course;
import static pl.ais.example.webapp.domain.model.QParticipant.participant;
...
final JPAQuery query = new JPAQuery(entityManager, jpqlTemplates).from(course).leftJoin(course.participants, participant).orderBy(participant.name.asc());
final SearchResults<String> searchResults = Results.forQuery(query).matching(course.participants.size().gt(5)).as(searchResults(participant.name));

The above code leads to following JPQL queries:

select count(course) from Course course left join course.participants as participant where size(course.participants) > 5
select participant.name from Course course left join course.participants as participant where size(course.participants) > 5 order by participant.name asc

Fetching first 10 course names, ordered by name

This time we use Selection for fetching only subset of matching records (ex. for paginating them on UI). Selection instances should be created with specific SelectionFactory. If you use Spring Framework in your application, you may define SelectionFactory as singleton:

<bean id="selectionFactory" class="pl.ais.commons.query.dsl.QuerydslSelectionFactory" />

and then inject as dependency into your code:

import static pl.ais.commons.query.dsl.Transformers.list;
import static pl.ais.example.webapp.domain.model.QCourse.course;
...
@Autowired
private transient QuerydslSelectionFactory selectionFactory;
....
final JPAQuery query = new JPAQuery(entityManager, jpqlTemplates).from(course);
final QuerydslSelection selection = selectionFactory.createSelection(0, 10, course.name.asc(), course.id.asc());
final List<String> searchResults = Results.forQuery(query).within(selection).as(list(course.name));

The above code leads to following JPQL query:

select course.name from Course course order by course.name asc, course.id asc

Above Selection usage example restricts returned results to first 10 records, ordered by course name. Please, note that we use course ID as the second ordering part, this is required to avoid problems with ordering data described in JPQL - pagination on Oracle Database with Hibernate - see also discussion on: DZone.