MongoDB output, updating arrays of objects

I'm pulling data from a few MySQL tables and moving them over to MongoDB in PDI 4.4. The structure of the MongoDB documents is like so:

{
Order: 12345
CustomerID: 23456
OrderItems: [
{
part: abc,
quantity: 4
},
{
part: def,
quantity: 5
}
]
}

I'm going to be pulling the same data from MySQL repeatedly, and I'd like to upsert to MongoDB. The problem is, the only way to get the array objects right is to do a $push operation to OrderItems[]. This means that the next time the transformation is run, the same to lines will be pushed onto the array, so the order shows four order items instead of two. This will continue to increase each time the transformation is run.

My first idea was to truncate the table each time the transformation is run. This is not going to work in my situation; truncating is not an option (the details of this are boring). MongoDB has a modifier called $unset, which would be ideal, but that appears to not be supported in PDI 4.4. My other idea was to put a null value in there, but the MongoDB Output step simply ignores nulls and empty strings when updating the database. Any other ideas? Thanks!