Support Maybe patterns for navigations

[ajcvickers]

See discussion here: https://github.com/dotnet/efcore/discussions/22364

Related: #240 #752

[alexmurari]

@ajcvickers

Just to give this issue a little bit of context:

The request here is to make navigation properties more flexible:

public class Student
{
    public Course FavoriteCourse { get; set; } // This could be null.
}

In functional programming, we are taught to be more explicit on whether or not a reference type object is null, so we wrap them in a Maybe<T> monad, that works more-or-less like Nullable<T>.

public class Student
{
    private Course _favoriteCourse; // Backing field remains of the same type.
    public Maybe<Course> FavoriteCourse => _favoriteCourse; // Maybe<T> provides implicit conversion.
}

Use it like this:

Maybe<Course> favCourseOrNothing = student.FavoriteCourse;

if (favCourseOrNothing.HasNoValue)
    return Error("Bad student doesn't have a favorite course");

Course favCourse = favCourseOrNothing.Value; // Throws exception if null, preventing us from walking around with null objects.

EF Core doesn't know how to convert Maybe<T> to T, so it gives an exception when building the model.
But, since EF Core works primarily on top of backing fields (and we actually map to the backing field), something like that is totally possible.

• References

https://github.com/vkhorikov/CSharpFunctionalExtensions#make-nulls-explicit-with-the-maybe-type
https://enterprisecraftsmanship.com/posts/functional-c-non-nullable-reference-types/

[lucasteles]

I think that having a way to define a NullValueConverter for navigations would be good enough