Doctrine: Check if relation exists for a Discriminator entity - php

Question

I'm trying to check if a relationship exists or not, in other words, is the column NULL or filled out.
Here I have my QueryBuilder:
$qb->select('v')
->from('Entities\Voucher', 'v')
->leftJoin('Entities\VoucherCopy', 'vc', \Doctrine\ORM\Query\Expr\Join::WITH, 'v.voucherCopy = vc.id')
->where("v.code LIKE :voucherCode")
->andWhere('vc.id IS NULL');
This would be ok if VoucherCopy wasn't Extending "Entities\Codes" - a Single Table Inheritance
VoucherCopy:
class VoucherCopy extends Codes
Codes:
/**
* Entities\DiscountCode
*
* #Table(name="discount_code")
* #Entity()
*
* #InheritanceType("SINGLE_TABLE")
* #DiscriminatorColumn(name="type", type="string")
* #DiscriminatorMap({"D" = "DiscountCode", "VT" = "PortalVoucher", "V" = "VoucherCopy"})
*/
class Codes
The problem is that the executed SQL from this query builder is:
SELECT v0_.*
FROM voucher v0_
LEFT JOIN discount_code d1_ ON (
v0_.discount_code_id = d1_.id
) WHERE (v0_.code LIKE '%DA70511911%' AND d1_.id IS NULL) AND d1_.type IN ('V')
;
Note the AND d1_.type IN ('V'). This is because DQL is automatically adding this because it knows that's the discriminator column for this Entity, but in doing so AND d1_.id IS NULL becomes redundant as I can't check if the relationship fails with the AND d1_.type IN ('V') check.
Is checking if a relationship exists whilst using single table inheritance possible?
I've tried leftJoin to Entities\Code (the parent) directly, but I can't add the discriminator column (type) as a property because you get error: Duplicate definition of column 'type' on entity 'Entities\Codes' in a field or discriminator column mapping.