score:2

Accepted answer
[ForeignKey("GroupId")]
public virtual Group Group { get; set; }

score:0

modelBuilder.Entity<GroupMetier>()
.HasRequired<Group>(gm => gm.Group)
.WithOptional(g => g.GroupMetier)
.Map(g => g.MapKey("GROUP_ID"));

score:1

[ForeignKey("GROUP_ID")]
public virtual Group Group { get; set; }

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