Webiny\Component\Entity\Attribute\Many2ManyAttribute::save PHP Method

    public function save()
    {
        $collectionName = $this->intermediateCollection;
        $firstClassName = $this->extractClassName($this->getParentEntity());
        $secondClassName = $this->extractClassName($this->getEntity());
        // Make sure indexes exist
        $indexOrder = [$firstClassName, $secondClassName];
        list($indexKey1, $indexKey2) = $this->arr($indexOrder)->sort()->val();
        $index = new CompoundIndex($collectionName, [$indexKey1 => 1, $indexKey2 => 1], false, true);
        Entity::getInstance()->getDatabase()->createIndex($collectionName, $index, ['background' => true]);
        /**
         * Insert values
         */
        $existingIds = [];
        $firstEntityId = $this->getParentEntity()->id;
        foreach ($this->getValue() as $item) {
            if ($item instanceof AbstractEntity && !$item->exists()) {
                $item->save();
            }
            if ($item instanceof AbstractEntity) {
                $secondEntityId = $item->id;
            } else {
                $secondEntityId = $item;
            }
            $existingIds[] = $secondEntityId;
            $data = [$firstClassName => $firstEntityId, $secondClassName => $secondEntityId];
            try {
                Entity::getInstance()->getDatabase()->insertOne($collectionName, $this->arr($data)->sortKey()->val());
            } catch (BulkWriteException $e) {
                // Unique index was hit and an exception is thrown - that's ok, means the values are already inserted
                continue;
            }
        }
        /**
         * Remove old links
         */
        $removeQuery = [$firstClassName => $firstEntityId, $secondClassName => ['$nin' => $existingIds]];
        Entity::getInstance()->getDatabase()->delete($collectionName, $removeQuery);
        /**
         * The value of many2many attribute must be set to 'null' to trigger data reload on next access.
         * If this is not done, we may not have proper links between the 2 entities and it may seem as if data was missing.
         */
        $this->setValue(null);
    }