在 Go 中推广 *sql.Rows 扫描

Question

我正在使用 Go 开发一个 Web API，并且有很多冗余的数据库查询扫描代码。

func (m *ContractModel) WorkQuestions(cid int) ([]models.WorkQuestion, error) {
    results, err := m.DB.Query(queries.WORK_QUESTIONS, cid)
    if err != nil {
        return nil, err
    }

    var workQuestions []models.WorkQuestion
    for results.Next() {
        var wq models.WorkQuestion
        err = results.Scan(&wq.ContractStateID, &wq.QuestionID, &wq.Question, &wq.ID, &wq.Answer, &wq.Compulsory)
        if err != nil {
            return nil, err
        }
        workQuestions = append(workQuestions, wq)
    }

    return workQuestions, nil
}

func (m *ContractModel) Questions(cid int) ([]models.Question, error) {
    results, err := m.DB.Query(queries.QUESTIONS, cid)
    if err != nil {
        return nil, err
    }

    var questions []models.Question
    for results.Next() {
        var q models.Question
        err = results.Scan(&q.Question, &q.Answer)
        if err != nil {
            return nil, err
        }
        questions = append(questions, q)
    }

    return questions, nil
}

func (m *ContractModel) Documents(cid int) ([]models.Document, error) {
    results, err := m.DB.Query(queries.DOCUMENTS, cid)
    if err != nil {
        return nil, err
    }

    var documents []models.Document
    for results.Next() {
        var d models.Document
        err = results.Scan(&d.Document, &d.S3Region, &d.S3Bucket, &d.Source)
        if err != nil {
            return nil, err
        }
        documents = append(documents, d)
    }

    return documents, nil
}

我需要概括这段代码，以便我可以将结果*sql.Rows 传递给一个函数并获得一个包含扫描行的结构切片。我知道 sqlx 包中有一个StructScan 方法，但由于我有大量使用 go 标准 database/sql 包编写的代码，因此无法使用该方法。

使用反射包，我可以创建一个通用的 StructScan 函数，但反射包不能从传递的 interface{} 类型创建一个结构切片。我需要实现的是如下

func RowsToStructs(rows *sql.Rows, model interface{}) ([]interface{}, error) {
    // 1. Create a slice of structs from the passed struct type of model
    // 2. Loop through each row,
    // 3. Create a struct of passed mode interface{} type
    // 4. Scan the row results to a slice of interface{}
    // 5. Set the field values of struct created in step 3 using the slice in step 4
    // 6. Add the struct created in step 3 to slice created in step 1
    // 7. Return the struct slice
}

我似乎找不到一种方法来扫描作为模型参数传递的结构并使用反射包创建一个切片。有什么解决方法吗？还是我看问题的方式有误？

结构字段具有从结果返回的正确列数和正确的顺序

Answer 1

您可以通过将指向目标切片的指针作为参数传递来避免在调用函数中使用类型断言。这是带有该修改的 RowsToStructs：

// RowsToStructs scans rows to the slice pointed to by dest.
// The slice elements must be pointers to structs with exported
// fields corresponding to the the columns in the result set.
//
// The function panics if dest is not as described above.
func RowsToStructs(rows *sql.Rows, dest interface{}) error {

    // 1. Create a slice of structs from the passed struct type of model
    //
    // Not needed, the caller passes pointer to destination slice.
    // Elem() dereferences the pointer.
    //
    // If you do need to create the slice in this function
    // instead of using the argument, then use
    // destv := reflect.MakeSlice(reflect.TypeOf(model).

    destv := reflect.ValueOf(dest).Elem()

    // Allocate argument slice once before the loop.

    args := make([]interface{}, destv.Type().Elem().NumField())

    // 2. Loop through each row

    for rows.Next() {

        // 3. Create a struct of passed mode interface{} type
        rowp := reflect.New(destv.Type().Elem())
        rowv := rowp.Elem()

        // 4. Scan the row results to a slice of interface{}
        // 5. Set the field values of struct created in step 3 using the slice in step 4
        //
        // Scan directly to the struct fields so the database
        // package handles the conversion from database
        // types to a Go types.
        //
        // The slice args is filled with pointers to struct fields.

        for i := 0; i < rowv.NumField(); i++ {
            args[i] = rowv.Field(i).Addr().Interface()
        }

        if err := rows.Scan(args...); err != nil {
            return err
        }

        // 6. Add the struct created in step 3 to slice created in step 1

        destv.Set(reflect.Append(destv, rowv))

    }
    return nil
}

这样称呼它：

func (m *ContractModel) Documents(cid int) ([]*models.Document, error) {
    results, err := m.DB.Query(queries.DOCUMENTS, cid)
    if err != nil {
        return nil, err
    }
    defer results.Close()
    var documents []*models.Document
    err := RowsToStruct(results, &documents)
    return documents, err
}

通过将查询移动到辅助函数来消除更多样板：

func QueryToStructs(dest interface{}, db *sql.DB, q string, args ...interface{}) error {
    rows, err := db.Query(q, args...)
    if err != nil {
        return err
    }
    defer rows.Close()
    return RowsToStructs(rows, dest)
}

这样称呼它：

func (m *ContractModel) Documents(cid int) ([]*models.Document, error) {
    var documents []*model.Document
    err := QueryToStructs(&documents, m.DB, queries.DOCUMENTS, cid)
    return documents, err
}

Answer 2

"...但是反射包无法从传递的接口{} 类型创建一个结构片段。" -- 你在找这个吗？

func sliceFromElemValue(v interface{}) (interface{}) {
    rt := reflect.TypeOf(v)
    rs := reflect.MakeSlice(reflect.SliceOf(rt), 0, 0)

    for i := 0; i < 3; i++ { // dummy loop
        rs = reflect.Append(rs, reflect.New(rt).Elem())
    }

    return rs.Interface()
}

https://play.golang.com/p/o4AJ-f71egw

---

“我需要实现的是如下”

func RowsToStructs(rows *sql.Rows, model interface{}) ([]interface{}, error) { ...

或者你正在寻找这个？

func sliceFromElemValue(v interface{}) ([]interface{}) {
    rt := reflect.TypeOf(v)
    s := []interface{}{}

    for i := 0; i < 3; i++ { // dummy loop
        s = append(s, reflect.New(rt).Elem().Interface())
    }

    return s
}

https://play.golang.com/p/i57Z8OO8n7G