How to use

create

method

in

com.mysema.query.group.QPair

/**
 * Create a new aggregating map expression using a backing TreeMap using the given comparator
 *
 * @param key
 * @param value
 * @param comparator
 * @return
 */
public static <K, V> AbstractGroupExpression<Pair<K, V>, SortedMap<K, V>> sortedMap(Expression<K> key, Expression<V> value, Comparator<? super K> comparator) {
  return GMap.createSorted(QPair.create(key, value), comparator);
}

/**
 * Create a new aggregating map expression using a backing TreeMap
 *
 * @param key
 * @param value
 * @return
 */
public static <K extends Comparable<? super K>, V> AbstractGroupExpression<Pair<K, V>, SortedMap<K, V>> sortedMap(Expression<K> key, Expression<V> value) {
  return GMap.createSorted(QPair.create(key, value));
}

/**
 * Create a new aggregating map expression using a backing LinkedHashMap
 *
 * @param key
 * @param value
 * @return
 */
public static <K, V, T, U> AbstractGroupExpression<Pair<K, V>, Map<T, U>> map(GroupExpression<K, T> key, GroupExpression<V, U> value) {
  return new GMap.Mixin<K, V, T, U, Map<T, U>>(key, value, GMap.createLinked(QPair.create(key, value)));
}

/**
 * Create a new aggregating map expression using a backing TreeMap
 *
 * @param key
 * @param value
 * @return
 */
public static <K extends Comparable<? super K>, V, T extends Comparable<? super T>, U> AbstractGroupExpression<Pair<K, V>, SortedMap<T, U>> sortedMap(GroupExpression<K, T> key, GroupExpression<V, U> value) {
  return new GMap.Mixin<K, V, T, U, SortedMap<T, U>>(key, value, GMap.createSorted(QPair.create(key, value)));
}

/**
 * Create a new aggregating map expression using a backing LinkedHashMap
 *
 * @param key
 * @param value
 * @return
 */
@WithBridgeMethods(value=Expression.class,castRequired=true)
public static <K, V> AbstractGroupExpression<Pair<K, V>,Map<K, V>> map(Expression<K> key, Expression<V> value) {
  return GMap.createLinked(QPair.create(key, value));
}

/**
 * Create a new aggregating map expression using a backing TreeMap using the given comparator
 *
 * @param key
 * @param value
 * @param comparator
 * @return
 */
public static <K, V, T, U> AbstractGroupExpression<Pair<K, V>, SortedMap<T, U>> sortedMap(GroupExpression<K, T> key, GroupExpression<V, U> value, Comparator<? super T> comparator) {
  return new GMap.Mixin<K, V, T, U, SortedMap<T, U>>(key, value, GMap.createSorted(QPair.create(key, value), comparator));
}

@SuppressWarnings({ "rawtypes", "unchecked" })
public Mixin(GroupExpression<K, T> keyExpression, GroupExpression<V, U> valueExpression, AbstractGroupExpression<Pair<T, U>, R> mixin) {
  super((Class) mixin.getType(), QPair.create(keyExpression.getExpression(), valueExpression.getExpression()));
  this.keyExpression = keyExpression;
  this.valueExpression = valueExpression;
  this.mixin = mixin;
}

@Test
@ExcludeIn({MYSQL, DERBY})
@NoBatooJPA
public void GroupBy() {
  QAuthor author = QAuthor.author;
  QBook book = QBook.book;
  for (int i = 0; i < 10; i++) {
    Author a = new Author();
    a.setName(String.valueOf(i));
    save(a);
    for (int j = 0; j < 2; j++) {
      Book b = new Book();
      b.setTitle(String.valueOf(i)+" "+String.valueOf(j));
      b.setAuthor(a);
      save(b);
    }
  }
  Map<Long, List<Pair<Long, String>>> map = query()
    .from(author)
    .join(author.books, book)
    .transform(GroupBy
      .groupBy(author.id)
      .as(GroupBy.list(QPair.create(book.id, book.title))));
  for (Entry<Long, List<Pair<Long, String>>> entry : map.entrySet()) {
    System.out.println("author = " + entry.getKey());
    for (Pair<Long,String> pair : entry.getValue()) {
      System.out.println("  book = " + pair.getFirst() + "," + pair.getSecond());
    }
  }
}