You're mixing up your variables. When you use merge syntax in SOQL, you must use the name of an existing variable in your scope. So if you had a variable called `stockId`, you could use: `'WHERE Id = :stockId'`. You can't do any cross-references, though, so `stock.Id` cannot be merged in.

However, if you use `'WHERE Id = :'' + stockId`, you get a resulting query string of `'WHERE Id = a0236000001bOXdAAM'`. Since you do not have a variable called `a0236000001bOXdAAM`, you get an error.