Update can add negative levels of stock. Negative stock handling requires error cases.

[BarDweller]

If the PUT call is for a stock not currently held by the identified portfolio, and the stock level specified is negative. The insert will succeed and store a negative quantity of stock.

Handling for negative amounts is never really clear, it should probably result in an error, and the request being rejected entirely if a request drives a stock to negative levels, as it would imply the portfolio was not in the state the user expected. (eg, tring to remove 2000 shares from a holding of 20 should result in silently dropping the 20 shares as a 'success' it should be an error that the other 1980 requested removals were not possible, and thus the request was aborted)