Do I need to configure both sides of a relationship with Entity Framework with Fluent API?

Question

I'm new to Fluent API. In my scenario, a Student can be in one Grade and a Grade can have many Students. Then, these two statements accomplish the same thing:

modelBuilder
.Entity<Student>()
.HasRequired<Grade>(s => s.Grade)
.WithMany(s => s.Students);

And:

modelBuilder
.Entity<Grade>()
.HasMany<Student>(s => s.Students)
.WithRequired(s => s.Grade);

My question is - how should I choose one statement over the other? Or do I need both statements?

Answer 1

For bidirectional relationship like yours (i.e. when both ends have navigation properties), it doesn't really matter, you can use one or the another (you can also use both, but it's not recommended because it's redundant and may lead to out of sync between the two).

It really matters when you have unidirectional relationship because only With methods have parameterless overloads.

Imagine you don't have Grade.Students property. Then you can use only:

modelBuilder.Entity<Student>()
    .HasRequired(s => s.Grade)
    .WithMany();

and if you don't have Student.Grade property, then you can use only:

modelBuilder.Entity<Grade>()
    .HasMany(s => s.Students)
    .WithRequired();

Answer 2

You just need one.This is more than enough for 1 : M relationship.

modelBuilder.Entity<Student>()
            .HasRequired<Grade>(s => s.Grade) //Student entity requires Grade 
            .WithMany(s => s.Students); //Grade entity includes many Students entities