A DbContext remembers every entity it has loaded or been told about, until you say otherwise. ChangeTracker.Clear() makes it forget all of them, including changes you haven't saved. That's the fix for one nasty retry bug, and useful in a couple of other places.
Could not save changes. Please configure your entity type accordingly. tells you nothing, and it isn't meant to be read on its own. The real error is one level down, with a number you can test in code. Where it sits depends on which method threw, so a small helper is worth having.
LINQ covers most queries, and now and then you need SQL of your own: FOR UPDATE, SELECT VERSION(), or a query LINQ makes awkward. EF Core has three methods for that, and which one you use depends on what you want back.
When two people buy the last item, the safe way to decide who gets it is to let the database decide, in one statement. Put the stock check in the UPDATE's WHERE, and read how many rows changed. That's all of it, and every round I ran gave one winner and four polite refusals.
If another transaction holds a row, MySQL makes your statement wait up to 50 seconds by default. For a web request, that's far too long. Here's how to set a shorter limit from EF Core, on every connection, without relying on CommandTimeout, which doesn't do this job with MySql.Data.
Some settings belong to a connection, not a statement: a time limit, a session variable, a time zone. EF Core has a hook for exactly that. A DbConnectionInterceptor runs your SQL each time a connection opens, and it's inexpensive enough not to think about.
A savepoint lets part of a transaction fail while the rest carries on. In EF Core, you create one with CreateSavepointAsync, and EF Core also creates its own around each SaveChanges inside a transaction. If you've seen __EFSavePoint in MySQL's log, that's where it comes from.
BeginTransactionAsync takes an isolation level as its first argument, and that's the whole API. The useful part is knowing what gets sent, how to confirm that MySQL really is using it, and which levels the driver refuses.
Sometimes a decision can't fit in one UPDATE: you need to read a row, think about it in C#, and only then change it, with nobody else changing it in the meantime. SELECT ... FOR UPDATE holds the row while you do that. EF Core has no LINQ method for it, so it takes FromSql.
When a connection drops during COMMIT, nobody knows whether it worked. ExecuteInTransactionAsync handles that moment: it begins, commits and retries for you, and when the commit's outcome is unknown, it calls a function you supply to find out. You still have to give that function something to look for.
