Join on Subquery with multiple columns

[Richard]

I have the following model (not actual, but enough for sample purposes):

class TransactionLine(models.Model):
   txn_id = models.IntegerField()
   entity = models.CharField(max_length=10, null=True, default=None)
   order = models.PositiveIntegerField()

what I'm trying to do is "find the first line in a transaction, that has a non-null entity when ordered by 'order'". In SQL (and django), I can use a group by to get which line order has the first non-null entity:

select
    txn_id,
    min(`order`)
from transactionline
where
    entity is not null
group by txn_id
order by txn_id, `order`

However, that query alone doesn't give me the entity for that line. This is where I get stuck in django, because in SQL I can do:

select
   t.txn_id, t.entity, t.order
from transactionline t
inner join (
    select
        txn_id,
        min(`order`) as ordering
    from transactionline
    where
        entity is not null
    group by txn_id
    order by txn_id, `order`
) t1 on t.txn_id=t1.txn_id AND t.order = t1.ordering

But I haven't found how I can join on a subquery in django that has multiple conditions in the ON clause.

Is there anyway to achieve this without dropping to raw sql?

aside: Using Django 2.1 (can't upgrade to 2.2 because we have a hard dependency on pymysql)

Thanks!