We are using EF Core with an existing database. We configured the data model in code. We accidentally had an incorrect relation configuration. We configured a one to one relation:

        builder
            .HasOne(entity => entity.Blog)
            .WithOne()                              // Misconfiguration
            .HasForeignKey<Post>(x => x.BlogId);    // Misconfiguration

This should have been a one to many:

        builder
            .HasOne(entity => entity.Blog)
            .WithMany()                          // Correct configuration
            .HasForeignKey(x => x.BlogId);       // Correct configuration

Because of the misconfiguration during a SELECT query some of the entities are already marked for deletion. When later SaveChanges is called these entities are deleted.

We would have expected in this scenario that EF Core would throw an error about incorrect database configuration instead of silently marking entities for deletion.

To reproduce:

• Verify/update path to the existing blogging.db in ctor of BloggingContext in Model.cs
• Run the program.
• It will query for 2 posts and after that it will log to the console that it retrieved 2 posts and one of them is marked for deletion.