[{_id:1,sku:"abc123",quantity:10,metrics:{orders:2,ratings:3.5}}]

db.collection.update({sku:"abc123"},[{$set:{quantity:{$cond:{if:{$lt:[{$add:["$quantity",-20]},0]},then:0,else:{$add:["$quantity",-20]}}},"metrics.orders":1}}])