[Query]
List<ValueTuple<Company, Staff>> Query();

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

    public string Name { get; set; }

    public List<ChildEntity> Children { get; set; }
}

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

    public string Name { get; set; }
}

accessor.Query().MapOneToMany(p => p.Id, (p, cs) => p.Children = cs)

// Helper
public static class NestedExtensions
{
    public static IEnumerable<TP> MapOneToMany<TP, TC, TPKey>(
        this IEnumerable<Tuple<TP, TC>> source,
        Func<TP, TPKey> parentKeySelector,
        Action<TP, List<TC>> combiner)
    {
        return MapOneToMany(
            source,
            x => x.Item1,
            x => x.Item2,
            parentKeySelector,
            EqualityComparer<TPKey>.Default,
            combiner);
    }

    public static IEnumerable<TP> MapOneToMany<T, TP, TC, TPKey>(
        this IEnumerable<T> source,
        Func<T, TP> parentSelector,
        Func<T, TC> childSelector,
        Func<TP, TPKey> parentKeySelector,
        IEqualityComparer<TPKey> keyComparer,
        Action<TP, List<TC>> combiner)
    {
        using (var en = source.GetEnumerator())
        {
            if (en.MoveNext())
            {
                var item = en.Current;
                var parent = parentSelector(item);
                var key = parentKeySelector(parent);
                var children = new List<TC>
                {
                    childSelector(item)
                };

                while (en.MoveNext())
                {
                    var newItem = en.Current;
                    var newParent = parentSelector(newItem);
                    var newKey = parentKeySelector(newParent);

                    if (!keyComparer.Equals(key, newKey))
                    {
                        combiner(parent, children);
                        yield return parent;

                        key = newKey;
                        children = new List<TC>();
                    }

                    children.Add(childSelector(newItem));
                }

                combiner(parent, children);
                yield return parent;
            }
        }
    }
}