code: func (f *Field) NotNull() *PQLRowQuery {
  	text := fmt.Sprintf("Range(%s != null)", f.name)
  	q := NewPQLRowQuery(text, f.index, nil)
  	q.hasKeys = f.options.keys || f.index.options.keys
  	return q
  }
docs: // NotNull creates a not equal to null query.