55 posts tagged with "MySQL"

A foreign key ties a column to another table: a product's CategoryId must be the Id of a real category. Before you delete a row, rename a table or change a column, you need to know which foreign keys are involved and what MySQL will do with them. This guide lists them three ways on a real MySQL 9.7 database: for one table, for the whole database, and in reverse, every table that points at a given one.

Indexes decide how fast MySQL can find rows, and which values must be unique. SHOW INDEX lists them for one table, and INFORMATION_SCHEMA.STATISTICS lists them for a whole database, ready to filter. This guide runs both on a real MySQL 9.7 database created by EF Core, and explains how to read multi-column indexes, where the order of the columns matters.

When you need to know exactly what a MySQL table looks like, which columns, which types, what can be empty and which keys exist, three statements answer at different levels of detail. DESCRIBE gives a quick table of columns, SHOW CREATE TABLE gives the complete definition with every index and constraint, and INFORMATION_SCHEMA.COLUMNS lets you query the answer. This guide runs all of them on a real MySQL 9.7 table created by EF Core.

The first thing to check in a MySQL database is which tables it has. SHOW TABLES answers that in one line, and INFORMATION_SCHEMA.TABLES adds the details: storage engine, collation, size and an approximate row count. This guide runs both against a real MySQL 9.7 database and explains the one number in there you shouldn't trust.

Unable to create a 'DbContext' of type 'Dewiride.Inventory.Data.InventoryDbContext'. The exception 'Unable to resolve service for type 'Microsoft.EntityFrameworkCore.DbContextOptions`1[Dewiride.Inventory.Data.InventoryDbContext]' while attempting to activate 'Dewiride.Inventory.Data.InventoryDbContext'.' was thrown while attempting to create an instance. The useful part is the line printed just before it: the app's own startup code had failed, so EF Core's tools couldn't get the DbContext the normal way. Here's how I ran into it with a migrations bundle, why --connection didn't help, and the fix.

NU1608: Detected package version outside of dependency constraint: Pomelo.EntityFrameworkCore.MySql 9.0.0 requires Microsoft.EntityFrameworkCore.Relational (>= 9.0.0 && <= 9.0.999) but version Microsoft.EntityFrameworkCore.Relational 10.0.14 was resolved. The message spells out the problem: the newest Pomelo MySQL provider is built for EF Core 9, and the project runs EF Core 10. That isn't a version to nudge; it's a provider that doesn't support your EF Core yet.

I gave a column a case-sensitive collation with EF Core's UseCollation("utf8mb4_0900_as_cs"), created the migration, applied it, and dotnet ef printed Done. The column was still utf8mb4_0900_ai_ci, and MySQL still refused desks as a duplicate of Desks. Oracle's MySQL provider for EF Core leaves UseCollation out of the SQL it generates. Here's how I found that, what Oracle documents instead, and the migration that does change the column.

The migration '20261103044430_MakeDescriptionRequired' has already been applied to the database. Revert it and try again. If the migration has been applied to other databases, consider reverting its changes using a new migration instead. dotnet ef migrations remove checks the database before it deletes anything, and this migration was in it. The message holds both fixes: one for your own development database, one for every other database.

Deleting a category through EF Core worked as designed until the request loaded the category's products first. Then the delete failed with System.InvalidOperationException: The association between entity types 'Category' and 'Product' has been severed, but the relationship is either marked as required or is implicitly required because the foreign key is not nullable. It came from Remove(), before any SQL ran, so the error handling around SaveChangesAsync never saw it.

Four different mistakes, one message: Microsoft.EntityFrameworkCore.DbUpdateException: Could not save changes. Please configure your entity type accordingly. A duplicate name, a negative price, a missing category and a blocked delete all produced it, through Oracle's MySQL provider for EF Core. Nothing was wrong with any entity type's configuration. The real reason sits one line lower, in the inner MySqlException, and its error number is what your code should handle.