Doctrine 2 manyToOne relationship issue - symfony

Question

I have two entities, invoices and sales tax. Recently my team has decided to upgrade to Symfony 2.3 from 2.1. We had to rewrite a lot of the ways we were doing queries because of this, and some entities that didn't have relationships needed to have them.
Before the update, my invoices and sales tax records were created by getting the transaction id date('U'); and setting up both with the same transaction id (both tables have a primary index id as well).
So you can imagine it looks like this:
Sales Tax: id, transaction_id, amount
Invoice: id, transaction_id, amount
So when I queried for them I just joined on the transaction id. Now joins aren't working without relationships, so I'm having to update this to have a relationship. But when I go to create an invoice or sales tax record, I get this error: Notice: Undefined index: transactionId in C:\folders\vendor\doctrine\orm\lib\Doctrine\ORM\Persisters\BasicEntityPersister.php line 539.
In the code the sales tax record is being created just fine, but when it goes to create the invoice it fails:
public function insertSalesTax($transactionId, $amount)
{
$tax = new SalesTax();
$tax->setAmount($amount);
$tax->setTransactionId($transactionId);
$this->entityManager->persist($tax);
$this->entityManager->flush();
return $tax;
}
That inserts, but then I take that tax record and try to create the invoice:
$invoice = new Invoice();
$em = $this->entityManager;
$invoice //omitted other invoice data
->setSalesTax($salesData['salesTax']);
$em->persist($invoice);
$em->flush();
Here's the relevant portions of my mapping in yml:
Bundle\Entity\Invoice:
type: entity
table: invoices
indexes:
transactionId:
columns: [ transaction_id ]
id:
id:
type: integer
generator: { strategy: AUTO }
manyToOne:
salesTax:
targetEntity: SalesTax
inversedBy: invoice
joinColumn:
name: transaction_id
referencedColumnName: transaction_id
And SalesTax:
Bundle\Entity\SalesTax:
type: entity
table: sales_taxes
indexes:
transactionId:
columns: [ transaction_id ]
id:
id:
type: integer
generator: { strategy: AUTO }
oneToMany:
invoice:
targetEntity: Invoice
mappedBy: salesTax
If you're wondering why oneToMany, that's because invoices are stored as individual line items. There may be many invoices with the same transaction ID. One transaction ID in the invoices table represents one order, each row only represents a line item. So this invoice entity will probably need a self-referencing relationship at some point.