An order has a list of lines, and two things can be wrong with it: the list itself, and any one of its items. AddValidation in .NET 10 checks both, and it tells the client exactly which item is wrong.
By default, an enum in a minimal API's JSON is a number, and "type": 0 tells a reader nothing. One line in Program.cs makes every endpoint use the names, in both directions. It's worth knowing exactly what it accepts on the way in, too.
An IExceptionHandler is one class that decides what clients see when an exception escapes an endpoint. It answers the ones it recognises, and leaves everything else alone. Here's a complete one, and the setting I add since .NET 10.
When MySQL is down, or a row is locked for too long, a 500 tells the client that your code has a bug and there's no point trying again. 503 Service Unavailable with Retry-After says the truth: not now, come back in a few seconds. An exception handler does it in one place, for the whole API.
To read a request header in a minimal API, add a parameter with [FromHeader]. One detail decides how it behaves: string? makes the header optional, and string makes it required, with a 400 if it's missing.
A client that gets no answer can't tell whether its POST worked. If it tries again, you can get two orders. An Idempotency-Key header fixes that: the client sends the same key each time, and the second request gets back what the first one made.
SumAsync adds up a column in the database, without loading the rows. Two things are worth seeing: what it returns when there are no rows, since SQL's SUM gives NULL there, and how to get a total per group.
To load several rows by their IDs, call Contains on the list inside your Where. It reads backwards the first time, and EF Core turns it into a plain SQL IN. An empty list is worth seeing too: it doesn't fail, and it doesn't return everything.
Most apps never need this, because one DbContext is enough. When you do have two, such as separate contexts for separate modules, they can share one transaction: they need the same connection, and the second has to be told about the transaction. Here it is, working and rolling back.
TransactionScope wraps code in a transaction without passing anything around: everything on the connection inside it joins in, and it commits only if you call Complete(). It works with EF Core and MySql.Data, with three things to get right. If you don't need its ambient behaviour, BeginTransactionAsync is simpler.
