empty result when using has() and whereHas() on mongodb relation

[sgimmai]

Laravel: v5.8.18
Jenssengers: v3.5.1
mongodb/mongodb: v1.4.2
php: v7.1.30

My code:

use Jenssegers\Mongodb\Eloquent\Model as Eloquent;
class AuditTrail extends Eloquent{
    protected $connection = 'mongodb';
    protected $collection = 'audit_trails';
    public $timestamps = [
        'created_at'
    ];
    ...
    public function purchaseorder()
    {
        return $this->hasMany('\PurchaseOrder', 'Purchase Order Number', 'Customer PO');
    }
    ...
}

use Jenssegers\Mongodb\Eloquent\Model as Eloquent;
class PurchaseOrder extends Eloquent{
    protected $connection = 'mongodb';
    protected $collection = 'purchase_orders';
    public $timestamps = [
        'created_at'
    ];
    ...
    public function audittrail()
    {
        return $this->hasMany('\AuditTrail', 'Customer PO', 'Purchase Order Number');
    }
    ...
}

PurchaseOrder::has('audittrail')->get() always returns an empty collection. I've tried swapping the relationship keys with the same results.

However, when querying on HybridRelationsPurchaseOrder::has('upload')->get() it works as expected.

[Smolevich]

@sgimmai, let's go to slack. Link here

[sgimmai]

using $this->belongsTo for both relations work.
is this the expected behaviour?

[cantacoop]

Laravel: v5.8.30
Jenssengers: v3.5.1
MongoDB 4.0.10 Community
php: v7.3.7

My code:

use Jenssegers\Mongodb\Eloquent\Model as Eloquent;
class Career extends Eloquent{
    protected $primaryKey = 'careerId';
    ...
}

use Jenssegers\Mongodb\Eloquent\Model as Eloquent;
use Illuminate\Database\Eloquent\Relations\BelongsTo;
class Candidate extends Eloquent{
    protected $primaryKey = 'candidateId';

    public function candidateCareer(): BelongsTo
    {
        return $this->belongsTo( Career::class, 'careerId' );
    }

    protected static function boot()
    {
        parent::boot();

        static::addGlobalScope( 'candidateCareer', function( Builder $builder ){
            $builder->has( 'candidateCareer' );
        } );
    }
    ...
}

Table careers:

| _id ObjectId             | careerId String | jobPosition String |
|--------------------------|-----------------|--------------------|
| 5d5a543a7d3ff806ac007722 | "1"             | Accounting         |
| 5d5a543a7d3ff806ac007724 | "2"             | Designer           |

Table candidates:

| _id ObjectId             | candidateId String | careerId String | name String |
|--------------------------|--------------------|-----------------|-------------|
| 5d5e33dc7d3ff81fa4006862 | "1"                | "2"             | Robert      |
| 5d5e34707d3ff81b64001ff9 | "2"                | "2"             | James       |

I always return an empty collection. I've tried change type careerId column string to integer on candidates table and using protected $casts = [ 'careerId' => 'string' ]; on Candidate class. It's work but not make sense for me. Can you provide a good solution to solve this problem? Thanks.