orm.go
func (f *Field) TopN(n uint64) *PQLRowQuery {
	q := NewPQLRowQuery(fmt.Sprintf("TopN(%s,n=%d)", f.name, n), f.index, nil)
	return q
}
func (f *Field) TopN(n uint64) *PQLRowQuery {
	q := NewPQLRowQuery(fmt.Sprintf("TopN(%s,n=%d)", f.name, n), f.index, nil)
	return q
}
// TopN creates a TopN query with the given item count.
// Returns the id and count of the top n rows (by count of columns) in the field.