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Entity Framework 4.1 RTW Code First - Does POCO one to many need reference to child entity and child entities primary key?
public class Parent
{
    public int Id
    {
        get;
        set;
    }

    public int ChildId
    {
        get;
        set;
    }

    public virtual Child Child
    {
        get;
        set;
    }
}

public class Child
{
    public int Id
    {
        get;
        set;
    }
}

public class ParentMapping : EntityTypeConfiguration<Parent>
{
    public ParentMapping()
    {
        HasKey(x => x.Id);

        HasRequired(X => x.Child)
            .WithMany()
            .Map(x => x.ToTable("Parent")
                        .MapKey("ChildId"));
    }
}

public class Parent
{
    public int Id
    {
        get;
        set;
    }

    public virtual Child Child
    {
        get;
        set;
    }
}

var parent = GetUpdatedParentSomehow();
// Dummy object for the old child if the relation is not loaded
parent.Child = new Child { Id = oldChildId };
// Attach the parent
context.Parents.Attach(parent);

// Create dummy for new child (you can also load a child from DB)
var child = new Child { ID = newChildId };
// No attach the child to the context so the context
// doesn't track it as a new child
context.Childs.Attach(child);
// Set a new child
parent.Child = child;
// Set parent as modified
context.Entry(parent).State = EntityState.Modified;
context.SaveChanges();