如何在Golang中为非本地结构添加接口？

huangapple
go评论106阅读模式
英文:

How to add interface to non-local struct in golang?

问题

我使用https://github.com/lib/pq从Postgres获取数据。
为了提取数据，我使用了一个带有protobuf Timestamp字段的第三方结构体，链接在这里：https://pkg.go.dev/google.golang.org/protobuf/types/known/timestamppb#Timestamp
所以问题是如何将time.Time的值转换为timestamppb.Timestamp并使Scan函数正常工作。

我尝试了将值扫描到实现了sql.Scanner接口的结构体中，这很简单。我只需像这样实现Scan函数：

  1. type Test struct {
  2. }
  4. func (t Test) Scan(src any) error {
  5.     // 转换值
  6. }

但是对于timestamppb.Timestamp这种非本地类型，这种方法不起作用。
然后我尝试定义本地类型并使用相同的技巧：

  1. type TimestampPb timestamppb.Timestamp
  3. func (t TimestampPb) Scan(src any) error {
  4.     // 转换值
  5. }

但是这个技巧也不起作用。而且我收到了警告：“'Scan'通过值传递锁：类型'TimestampPb'包含'protoimpl.MessageState'包含'sync.Mutex'，它是'sync.Locker'”。
如果我为TimestampPb指定指针，这个技巧也不起作用：

  1. func (t *TimestampPb) Scan(src any) error {
  2.     // 转换值
  3. }

所以我想知道，如何使timestamppb.Timestamp成为sql.Scanner的实例。这可能吗？

更新1
我尝试了一个小例子：

  1. type TimestampPb timestamppb.Timestamp
  3. type test struct {
  4.     date *timestamppb.Timestamp
  5. }
  7. func (s *Storage) test() {
  8.     rows, _ := s.db.Query("SELECT \"test_date\" FROM \"test_table\" LIMIT 1")
  10.     defer func() {
  11.         if rows != nil {
  12.             _ = rows.Close()
  13.         }
  14.     }()
  16.     for rows.Next() {
  17.         //var value *TimestampPb //可以工作
  18.         //var value TimestampPb //可以工作
  19.         //var value *timestamppb.Timestamp //不起作用
  20.         //testStruct := test{} //不起作用
  22.         //err := rows.Scan(&value) //扫描到变量
  23.         //err := rows.Scan(&testStruct.date) //扫描到结构体中的字段
  24.         if err != nil {
  25.             log.Fatalln(err)
  26.         }
  27.     }
  28. }
  30. func (tpb *TimestampPb) Scan(src any) error {
  31.     return nil
  32. }

我想知道是否可以使用testStruct的情况？

英文:

I use https://github.com/lib/pq for getting data from postgres.
For extracting data I use third-party struct, which has field with protobuf Timestamp https://pkg.go.dev/google.golang.org/protobuf/types/known/timestamppb#Timestamp
So
So the issue is make work Scan from time.Time to timestamppb.Timestamp

  1. type Stuff struct { //this struct non-local, this is from third-party package
  2.   Date *timestamppb.Timestamp
  3. }
  5. stuff = Stuff{}
  7. scanErr := rows.Scan(&stuff.Date)

I tried to Scan to struct witch implements sql.Scanner interface. That was easy. I just implement Scan function like this:

  1. type Test struct {
  2. }
  5. func (t Test) Scan(src any) error {
  6. //convert value
  7. }

but it doesn't work with timestamppb.Timestamp because it is non-local type.
So then I tried to define local type and do the same trick

  1. type TimestampPb timestamppb.Timestamp
  3. func (t TimestampPb) Scan(src any) error {
  4. //convert value
  5. }

but this trick didn't work. Moreover I have warning "'Scan' passes a lock by the value: type 'TimestampPb' contains 'protoimpl.MessageState' contains 'sync.Mutex' which is 'sync.Locker'"
This either doesn't work if I specify pointer for TimestampPb

  1. func (t *TimestampPb) Scan(src any) error {

So I wonder, how can I make timestamppb.Timestamp instance of sql.Scanner. Is it possible?

Update 1
Little example that I tried

  1. type TimestampPb timestamppb.Timestamp
  3. type test struct { //third-party struct, I can't modify it
  4. 	date *timestamppb.Timestamp
  5. }
  7. func (s *Storage) test() {
  8. 	rows, _ := s.db.Query("SELECT \"test_date\" FROM \"test_table\" LIMIT 1")
  10. 	defer func() {
  11. 		if rows != nil {
  12. 			_ = rows.Close()
  13. 		}
  14. 	}()
  16. 	for rows.Next() {
  17. 		//var value *TimestampPb //works
  18. 		//var value TimestampPb //works
  19. 		//var value *timestamppb.Timestamp //doesn't work
  20. 		//testStruct := test{} //doesn't work
  22. 		//err := rows.Scan(&value) //scan to variable
  23. 		//err := rows.Scan(&testStruct.date) //scan to field in the struct
  24. 		if err != nil {
  25. 			log.Fatalln(err)
  26. 		}
  27. 	}
  28. }
  30. func (tpb *TimestampPb) Scan(src any) error {
  31. 	return nil
  32. }

I wonder if I can use case with the testStruct?

答案1

得分: 0

pq库不支持*timestamppb.Timestamp作为时间戳类型。请参阅文档中支持的日期类型。

需要使用不同的类型进行扫描。

通常我会在函数中使用辅助类型来实现。例如：

  1. type row struct {
  2.   Date *timestamppb.Timestamp
  3.   // ... 其他字段
  4. }
  6. func Query() ([]row, error) {
  7.     rows, err := s.db.Query(...)
  8.   if err != nil {
  9.     return nil, err
  10.   }
  11.   defer rows.Close()
  13.   var rows []row
  14.   for rows.Next() {
  15.     type aux struct {
  16.       Date time.Time
  17.       // ... 其他字段
  18.     }
  19.     var value aux
  20.     if err := rows.Scan(&value); err != nil {
  21.       return nil, err
  22.     }
  23.     rows = append(rows, row{
  24.       Date: timestamppb.New(value.Date),
  25.       // ... 其他字段
  26.     }
  27.   }
  28.   return rows, nil
  29. }

希望这可以帮助到你！

英文:

The pq library doesn't support *timestamppb.Timestamp as a timestamp type. See the supported date types in the documentation.

A different type will need to be for scanning.

Often I do that by using an auxiliary type in the function. For example:

  1. type row struct {
  2.   Date *timestamppb.Timestamp
  3.   // ... more fields here
  4. }
  6. func Query() ([]row, error) {
  7.     rows, err := s.db.Query(...)
  8.   if err != nil {
  9.     return nil, err
  10.   }
  11.   defer rows.Close()
  13.   var rows []row
  14.   for rows.Next() {
  15.     type aux struct {
  16.       Date time.Time
  17.       // ... more fields here
  18.     }
  19.     var value aux
  20.     if err := rows.Scan(&value); err != nil {
  21.       return nil, err
  22.     }
  23.     rows = append(rows, row{
  24.       Date: timestamppb.New(value.Date),
  25.       // ... more fields here
  26.     }
  27.   }
  28.   return rows, nil
  29. }

huangapple
  • 本文由 发表于 2023年3月17日 05:28:20
  • 转载请务必保留本文链接：https://go.coder-hub.com/75761792.html
  • go
  • interface
  • pq
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