I created a migration, and the very next command said The migration 'AddProductVersion' was not found. The file was right there in the Migrations folder. The cause was a flag I'd added to save a few seconds: --no-build makes the EF Core tools use the last compiled DLL, and that DLL was older than the migration.
An UPDATE in MySQL stopped with ERROR 1175 (HY000): You are using safe update mode and you tried to update a table without a WHERE that uses a KEY column. Safe-updates mode refused it because nothing limited it to particular rows. Usually that's the mode doing what it's for, and the right fix is a better WHERE.
An INSERT into MySQL stopped with ERROR 1364 (HY000): Field 'CreatedAtUtc' doesn't have a default value. The column needs a value, the statement didn't give one, and MySQL has nothing to fall back on. The fix is to send the value, or give the column a default.
I sent a product price of 9.999 to my API, and it answered 201 Created with a price of 10.00. No error, no warning: MySQL rounded the amount to fit its DECIMAL(12,2) column, even in strict mode, and raised only a note that EF Core never shows. For money, silently changing the number isn't acceptable, so the API now refuses it.
A long category name made my API answer 500, with Data too long for column 'Name' at row 1 in the log. MySQL was right to refuse it: the column is varchar(100), and the name had 101 characters. The problem was that the request got as far as MySQL at all. A validation attribute stops it first, and the client gets an answer it can act on.
Right after I added a property to an EF Core entity, every request that touched that table answered 500, and the log said MySqlException: Unknown column 'p.Version' in 'field list'. The code built without a problem, because the C# was fine. The table simply didn't have that column yet: changing the class doesn't change the database until a migration does.
A check on this site refused two blog posts with front matter does not parse, and then missed comma between flow collection entries. Every line in those lists ended with a comma. The problem was a # in one post and [] in the other. Both mean something to YAML, even inside a list of plain words.
A Python script stopped with ModuleNotFoundError: No module named 'PIL'. The obvious fix, pip install PIL, fails as well, because the package isn't called that: you import PIL, and you install Pillow. Here's how the error came up, what the failing fix looks like, and the install that works.
ImageMagick's montage stopped with montage: unable to read font, and the contact sheet it left behind had no labels. This Homebrew build of ImageMagick has no list of fonts, so it had nothing to draw the labels with. Naming a font file with -font, before the images, fixes it.
Part 1 connected a .NET C# Web API to MySQL, but the database is still empty. This part describes the inventory data as C# classes and lets EF Core migrations create the matching MySQL tables, with the column types, keys, indexes and constraints you'd otherwise write by hand. You'll read the SQL before it runs, apply it, change the model and add a second migration, and then check every table from inside MySQL.
