Helper iterator and generator for one-to-many joins.

When you want to fetch a one-to-many relation, you're probably using a JOIN to avoid the N+1 selects problem.

Though, it may be difficult to iterate over the result, especially when you need the whole "many" part of the relation loaded for a process.

This is why I created this tiny library. It takes a Traversable of arrays, having a common key to distinguish the "one" part of the one-to-many relation, and sorted on this key (so it can yield items in streaming, without loading the whole set in memory).

It will then aggregate the "many" part of the relation in a configurable key.

composer require val/one-to-many-iterator

Your database result iterator, once converted into an array, looks like this (a typical JOIN):

<?php

[
    ['id' => 1, 'parent_column' => 'hello', 'child_column' => 'foo'],
    ['id' => 1, 'parent_column' => 'hello', 'child_column' => 'bar'],
    ['id' => 2, 'parent_column' => 'world', 'child_column' => 'baz'],
];

But you'd like to iterate over something like this:

<?php

[
    [
        'id' => 1,
        'parent_column' => 'hello',
        'children' => [
            ['child_column' => 'foo'],
            ['child_column' => 'bar'],
        ],
    ],
    [
        'id' => 2,
        'parent_column' => 'world',
        'children' => [
            ['child_column' => 'baz'],
        ],
    ],
];

To achieve this, just pass your database result to Val\Iterator\OneToManyIterator or Val\Iterator\OneToManyGenerator, while configuring the common key (here, id), and aggregate key (here, children).

Assuming $result contains the raw SQL result iterator:

<?php

// With an iterator
$aggregated = new OneToManyIterator('id', 'children', $result);

// With a generator
$aggregated = new OneToManyGenerator('id', 'children', $result);

foreach ($aggregated as $i => $parent) {
    $parent['id'];
    $parent['parent_column'];

    foreach ($parent['children'] as $child) {
        $child['child_column'];
    }
}

The difference between the iterator and the generator, is, well.. that the former implements a raw PHP iterator while the latter uses a PHP generator (available since version 5.5).

• When using a LEFT JOIN instead of a JOIN, thus not guaranteeing the presence of at least one relation item, the aggregate field will still contain one item of null values.