datastore/query.go
func (q *Query) toProto(dst *pb.Query, appID string) error {
	if len(q.projection) != 0 && q.keysOnly {
		return errors.New("datastore: query cannot both project and be keys-only")
	}
	if len(q.distinctOn) != 0 && q.distinct {
		return errors.New("datastore: query cannot be both distinct and distinct-on")
	}
	dst.Reset()
	dst.App = proto.String(appID)
	if q.kind != "" {
		dst.Kind = proto.String(q.kind)
	}
	if q.ancestor != nil {
		dst.Ancestor = keyToProto(appID, q.ancestor)
		if q.eventual {
			dst.Strong = proto.Bool(false)
		}
	}
	if q.projection != nil {
		dst.PropertyName = q.projection
		if len(q.distinctOn) != 0 {
			dst.GroupByPropertyName = q.distinctOn
		}
		if q.distinct {
			dst.GroupByPropertyName = q.projection
		}
	}
	if q.keysOnly {
		dst.KeysOnly = proto.Bool(true)
		dst.RequirePerfectPlan = proto.Bool(true)
	}
	for _, qf := range q.filter {
		if qf.FieldName == "" {
			return errors.New("datastore: empty query filter field name")
		}
		p, errStr := valueToProto(appID, qf.FieldName, reflect.ValueOf(qf.Value), false)
		if errStr != "" {
			return errors.New("datastore: bad query filter value type: " + errStr)
		}
		xf := &pb.Query_Filter{
			Op:       operatorToProto[qf.Op],
			Property: []*pb.Property{p},
		}
		if xf.Op == nil {
			return errors.New("datastore: unknown query filter operator")
		}
		dst.Filter = append(dst.Filter, xf)
	}
	for _, qo := range q.order {
		if qo.FieldName == "" {
			return errors.New("datastore: empty query order field name")
		}
		xo := &pb.Query_Order{
			Property:  proto.String(qo.FieldName),
			Direction: sortDirectionToProto[qo.Direction],
		}
		if xo.Direction == nil {
			return errors.New("datastore: unknown query order direction")
		}
		dst.Order = append(dst.Order, xo)
	}
	if q.limit >= 0 {
		dst.Limit = proto.Int32(q.limit)
	}
	if q.offset != 0 {
		dst.Offset = proto.Int32(q.offset)
	}
	if q.count != 0 {
		dst.Count = proto.Int32(q.count)
	}
	dst.CompiledCursor = q.start
	dst.EndCompiledCursor = q.end
	dst.Compile = proto.Bool(true)
	return nil
}
func (q *Query) toProto(dst *pb.Query, appID string) error {
	if len(q.projection) != 0 && q.keysOnly {
		return errors.New("datastore: query cannot both project and be keys-only")
	}
	if len(q.distinctOn) != 0 && q.distinct {
		return errors.New("datastore: query cannot be both distinct and distinct-on")
	}
	dst.Reset()
	dst.App = proto.String(appID)
	if q.kind != "" {
		dst.Kind = proto.String(q.kind)
	}
	if q.ancestor != nil {
		dst.Ancestor = keyToProto(appID, q.ancestor)
		if q.eventual {
			dst.Strong = proto.Bool(false)
		}
	}
	if q.projection != nil {
		dst.PropertyName = q.projection
		if len(q.distinctOn) != 0 {
			dst.GroupByPropertyName = q.distinctOn
		}
		if q.distinct {
			dst.GroupByPropertyName = q.projection
		}
	}
	if q.keysOnly {
		dst.KeysOnly = proto.Bool(true)
		dst.RequirePerfectPlan = proto.Bool(true)
	}
	for _, qf := range q.filter {
		if qf.FieldName == "" {
			return errors.New("datastore: empty query filter field name")
		}
		p, errStr := valueToProto(appID, qf.FieldName, reflect.ValueOf(qf.Value), false)
		if errStr != "" {
			return errors.New("datastore: bad query filter value type: " + errStr)
		}
		xf := &pb.Query_Filter{
			Op:       operatorToProto[qf.Op],
			Property: []*pb.Property{p},
		}
		if xf.Op == nil {
			return errors.New("datastore: unknown query filter operator")
		}
		dst.Filter = append(dst.Filter, xf)
	}
	for _, qo := range q.order {
		if qo.FieldName == "" {
			return errors.New("datastore: empty query order field name")
		}
		xo := &pb.Query_Order{
			Property:  proto.String(qo.FieldName),
			Direction: sortDirectionToProto[qo.Direction],
		}
		if xo.Direction == nil {
			return errors.New("datastore: unknown query order direction")
		}
		dst.Order = append(dst.Order, xo)
	}
	if q.limit >= 0 {
		dst.Limit = proto.Int32(q.limit)
	}
	if q.offset != 0 {
		dst.Offset = proto.Int32(q.offset)
	}
	if q.count != 0 {
		dst.Count = proto.Int32(q.count)
	}
	dst.CompiledCursor = q.start
	dst.EndCompiledCursor = q.end
	dst.Compile = proto.Bool(true)
	return nil
}
// toProto converts the query to a protocol buffer.