The reason is explained in the Loading Related Data section of the EF Core documentation.
The first behavior is because EF Core currently does not support lazy loading, so normally you’ll get
null for navigation properties until you specifically load them via eager or explicit loading. However, the Eager loading section contains the following:
Entity Framework Core will automatically fix-up navigation properties to any other entities that were previously loaded into the context instance. So even if you don’t explicitly include the data for a navigation property, the property may still be populated if some or all of the related entities were previously loaded.
which explains why the navigation property is not null in the second case.
Now, I’m not sure which of the two behaviors do you want to fix, so will try to address both.
The first behavior can be “fixed” by using one of the currently available methods for loading related data, for instance eager loading:
var mutants = db.Mutants.Include(m => m.OriginalCode).ToList();
The second behavior is “by design” and cannot be controlled. If you want to avoid it, make sure to use fresh new
DbContext instance just for executing a single query to retrieve the data needed, or use no tracking query.
Update: Starting with v2.1, EF Core supports Lazy Loading. However it’s not enabled by default, so in order to utilize it one should mark all navigation properties
virtual, install Microsoft.EntityFrameworkCore.Proxies and enable it via
UseLazyLoadingProxies call, or utilize Lazy-loading without proxies – both explained with examples in the EF Core documentation.