This gist brings join functionality to Doctrine Mongo ODM framework.

MongoJoin.php

<?php

/*
 * Copyright (c) 2016, Viktor Sidochenko <viktor.sidochenko@gmail.com>
 * All rights reserved.
 *
 *                           BSD License 
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are met:
 *   * Redistributions of source code must retain the above copyright
 *     notice, this list of conditions and the following disclaimer.
 *   * Redistributions in binary form must reproduce the above copyright
 *     notice, this list of conditions and the following disclaimer in the
 *     documentation and/or other materials provided with the distribution.
 *   * Neither the name of the <organization> nor the
 *     names of its contributors may be used to endorse or promote products
 *     derived from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED 
 * WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
 * DISCLAIMED. IN NO EVENT SHALL <COPYRIGHT HOLDER> BE LIABLE FOR ANY
 * DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
 * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
 * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
 * ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
 * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 * 
 */

namespace Util;

use MongoId;
use ReflectionClass;

/**
 * Description of MongoJoin
 *
 * @author sidochenko
 */
class MongoJoin {

    /**
     * Metod helps to join referenced objects in mongo ODM
     * @param array $parentResult result set where reference started
     * @param string $joinBy join field in $parentResult
     * @param array $referencedDocuments array of referenced documents
     * @param mixed $joinOwner document where $joinBy is primary key
     * @return mixed modified $parentresult
     *
     * Example:
     * $joinedInteractions = (new MongoJoin())->join($interactions->toArray(),
     * 'accountId',
     * [
     *      "account" => ['_id', 'ODM\Documents\Account'], //Key "account" is optional, if not set - collection name will used (see next string)
     *      ['accountId', 'ODM\Documents\User']
     * ]
     * , 'ODM\Documents\Account');
     */
    public function join($parentResult, $joinBy, $referencedDocuments, $joinOwner) {
        $dm = Application::getDocumentManager();
        $joinValues = [];

        //Parse join values from parend result
        array_walk($parentResult, function (&$item) use (&$joinValues, $joinBy) {
            array_push($joinValues, new MongoId($item->$joinBy));
        });

        //For each element of parent result
        foreach ($parentResult as $parentOne) {
            //For each referenced document
            foreach ($referencedDocuments as $referencedAlias=>$reference) {

                $referencedDocument=$reference[1];

                $field = $joinBy;
                $refField = $reference[0];

                if ($referencedDocument === $joinOwner) {
                    $refField = 'id';
                }

                //Find documents by referenced field
                $items = $dm->createQueryBuilder($referencedDocument)
                                ->select()->field($refField)->in($joinValues)->getQuery()->execute();
                $items = $items->toArray(FALSE);

                //Modify parent document element
                array_walk($items, function (&$item) use ($parentOne, $field, $refField, $referencedAlias) {

                    $className = new ReflectionClass(get_class($item));
                    if (is_int($referencedAlias)) {
                        $objToStore = strtolower($className->getShortName());
                    } else {
                        $objToStore=$referencedAlias;
                    }
                    if ($item->{$refField} === $parentOne->{$field}) {
                        $parentOne->{$objToStore} = $item;
                    }
                });
            }
        }
        return $parentResult;
    }

}

This is concept only. And can have performance issues on large data.
But it can be easy improved to pass all of parent results as search criteria to referenced documents.