This makes the following project possible:
python
web_order = WebOrder.one(Q._id == ObjectID(...), projection={
'items': {
'$sub': Item,
'product': {
'$ref': Product,
'name': True
}
}
})
In the example above we select a single web order (Frame) and map all items for the order to an item (SubFrame). Each item has a reference to a product (Frame) which we select limiting the projection to just the product's name.