db={"orders":[{"products":[{"productId":{"$oid":"00000001f1438712ca040fca"},}],},],"products":[{"name":"Apron","_id":{"$oid":"00000001f1438712ca040fca"},}]}

db.orders.aggregate([{"$lookup":{"from":"products","localField":"products.0.productId","foreignField":"_id","as":"firstProduct"}},{"$match":{"firstProduct.0.name":"Apron"}},{"$group":{"_id":null,"numOrders":{$sum:1}}}])