Aggregation framework: get related object

[NikitaKharkov]

At first my code and examples of data:

event: {
    "_id" : 3,
    "name": "someTestName",
    ...,
    "seoBlock" : {
        "$ref" : "event_seo_block",
        "$id" : 1004
    }
}

event_seo_block: {
    "_id": '1004',
    "text": "someText",
    "image": "http://lorempixel.com/640/480/?34447"
}

Fetch code:

$builder =  $this->getDocumentManager()->getDocumentCollection('AppBundle:Event')->createAggregationBuilder();
$builder
            ->match()
                ->field('status')->equals(Event::STATUS_ONLINE)
                ->field('leadingTradeFair')->equals(true)
                ->field('eventTo')->gte($this->getStartOfToday())
                ->field('seoBlock.$id')->in($validSeoBlocksIds)
            ->lookup('event_seo_block')
                ->localField('seoBlock.$id')
                ->foreignField('_id')
                ->alias('seoBlock')
            ->group()
                ->field('_id')->expression('$fairId')
                ->field('events')->push('$$ROOT')
            ->project()
                ->excludeIdField()
                ->field('seo')->expression('$events.seoBlock')
                ->field('events')->expression('$events')
                ->field('order')->add(null, null)
            ->sort('events.fairId', 1)

Explanation of problem:

I have event table with relation event_seo_block table as 1:1 . So when I made all the operations I have to get all info from related object, but I can't do it, because I can't point in expression seoBlock.$id (->localField('seoBlock.$id') string in code), I have an error:

FieldPath field names may not start with '$'.

I read about this problem, here I found about problem: jira.mongodb.org. But ODM don't support objectToArray and I can't solve problem in this way. Other links says the same.

I want to know is there a way how can I get this objects by aggregation framework?

What you do advise to do? Thanks

[alcaeus]

As I've mentioned in the original ticket, you're out of luck:

1. MongoDB does not support their own references in aggregation pipeline operators due to the leading $. This is a limitation we document for aggregation builder
2. If you are using ODM 1.2.0 or later, you can store references with the storeAs="ref" option which is similar to dbref but does not have a leading $ in nested fields.
3. $objectToArray is not supported in aggregation builder as we haven't gotten around to adding the features added in MongoDB 3.6. These will (most likely) be added in ODM 2.1.0, which will be started once we've finished the transition from ext-mongo to ext-mongodb.

The only option you have is injecting the expression manually, but beware I haven't tested this:

$builder->addFields()
    ->field('array')
    ->expression(['$objectToArray' => '$object'])

[NikitaKharkov]

Thanks for answer. I absolutely sure that second variant is worked, but we've already have structure on production, so I can't change it...
About first point: I want to know can we somehow come round this limitation in mongoDB in future? i think it's very good feature to lookup in such way?
About third point: unfortunately we have 3.2 image only and now to update it it will take some extra time which we can't allow to us.

[alcaeus]

As for the first point, you've already linked the ticket in the MongoDB issue tracker. All we can do is wait.

Since you're on MongoDB 3.2, having objectToArray in aggregation builder won't do you any good as your MongoDB server will just reject it.

[stale]

This issue has been automatically marked as stale because it has not had any recent activity. It will be closed in a week if no further activity occurs. Thank you for your contributions.