[英]Generalizing *sql.Rows Scan in Go
我正在使用 Go 开发 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
}
我需要概括此代码,以便可以将结果*sql.Rows
传递给 function 并获得包含扫描行的结构切片。 我知道sqlx package 中有一个StructScan
方法,但这不能使用,因为我有大量使用 go 标准数据库/sql ZEFE90A8E604A7C840E88D03A67F 编写的代码。
使用反射 package,我可以创建通用 StructScan function 但反射 package 类型无法从传递的接口创建结构切片{} 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 model 参数传递的结构并使用反射 package 创建一个切片。 是否有任何解决方法,或者我是否以错误的方式看待这个问题?
结构字段从结果返回的列数正确且顺序正确
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 function 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
}
像这样称呼它:
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
}
通过将查询移动到帮助程序 function 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)
}
像这样称呼它:
func (m *ContractModel) Documents(cid int) ([]*models.Document, error) {
var documents []*model.Document
err := QueryToStructs(&documents, m.DB, queries.DOCUMENTS, cid)
return documents, err
}
“......但反映 package 无法从传递的 interface{} 类型创建结构切片。” ——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所示”
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
}
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