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July 12, 2024 12:57
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Dynamic scan you query DB to struct golang
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| package dynscan | |
| import ( | |
| "database/sql" | |
| "fmt" | |
| "reflect" | |
| "strings" | |
| "time" | |
| "github.com/google/uuid" | |
| ) | |
| // DynamicScan scans a single row or multiple rows into the provided destination struct or slice | |
| func DynamicScan(db *sql.DB, result any, tableName string, dest any) error { | |
| switch res := result.(type) { | |
| case *sql.Row: | |
| columns, err := fetchColumns(db, tableName) | |
| if err != nil { | |
| return fmt.Errorf("error fetching columns: %w", err) | |
| } | |
| return scanRow(res, columns, dest) | |
| case *sql.Rows: | |
| columns, err := res.Columns() | |
| if err != nil { | |
| return fmt.Errorf("error fetching columns from rows: %w", err) | |
| } | |
| return scanRows(res, columns, dest) | |
| default: | |
| return fmt.Errorf("unsupported result type %T", result) | |
| } | |
| } | |
| // scanRow scans a single row into the provided destination struct | |
| func scanRow(row *sql.Row, columns []string, dest any) error { | |
| fieldPtrs, fieldMap, err := prepareFieldPointers(dest, columns) | |
| if err != nil { | |
| return err | |
| } | |
| if err := row.Scan(fieldPtrs...); err != nil { | |
| return fmt.Errorf("error scanning row: %w", err) | |
| } | |
| if err := setStructFields(dest, columns, fieldPtrs, fieldMap); err != nil { | |
| return fmt.Errorf("error setting struct fields: %w", err) | |
| } | |
| return nil | |
| } | |
| // scanRows scans multiple rows into the provided destination slice | |
| func scanRows(rows *sql.Rows, columns []string, dest any) error { | |
| destVal := reflect.ValueOf(dest) | |
| if destVal.Kind() != reflect.Ptr || destVal.Elem().Kind() != reflect.Slice { | |
| return fmt.Errorf("dest must be a pointer to a slice") | |
| } | |
| destSlice := destVal.Elem() | |
| elemType := destSlice.Type().Elem() | |
| for rows.Next() { | |
| elem := reflect.New(elemType).Elem() | |
| fieldPtrs, fieldMap, err := prepareFieldPointers(elem.Addr().Interface(), columns) | |
| if err != nil { | |
| return err | |
| } | |
| if err := rows.Scan(fieldPtrs...); err != nil { | |
| return fmt.Errorf("error scanning rows: %w", err) | |
| } | |
| if err := setStructFields(elem.Addr().Interface(), columns, fieldPtrs, fieldMap); err != nil { | |
| return fmt.Errorf("error setting struct fields: %w", err) | |
| } | |
| destSlice.Set(reflect.Append(destSlice, elem)) | |
| } | |
| return rows.Err() | |
| } | |
| // prepareFieldPointers prepares pointers for fields based on the column names | |
| func prepareFieldPointers(dest any, columns []string) ([]interface{}, map[string]int, error) { | |
| v := reflect.ValueOf(dest).Elem() | |
| t := v.Type() | |
| fieldPtrs := make([]interface{}, len(columns)) | |
| fieldMap := make(map[string]int) | |
| for i := 0; i < t.NumField(); i++ { | |
| field := t.Field(i) | |
| fieldMap[cleanTag(field.Tag.Get("json"))] = i | |
| } | |
| for i, column := range columns { | |
| idx, ok := fieldMap[column] | |
| if !ok { | |
| fieldPtrs[i] = new(interface{}) // Unmapped columns | |
| continue | |
| } | |
| field := v.Field(idx) | |
| fieldType := field.Type() | |
| switch fieldType { | |
| case reflect.TypeOf(time.Time{}): | |
| fieldPtrs[i] = new(sql.NullTime) | |
| case reflect.TypeOf(bool(false)): | |
| fieldPtrs[i] = new(sql.NullBool) | |
| case reflect.TypeOf(""): | |
| fieldPtrs[i] = new(sql.NullString) | |
| case reflect.TypeOf(uuid.UUID{}): | |
| fieldPtrs[i] = new(sql.NullString) | |
| case reflect.TypeOf([]byte{}): | |
| fieldPtrs[i] = new([]byte) | |
| default: | |
| if fieldType.Kind() == reflect.Ptr { | |
| fieldPtrs[i] = reflect.New(fieldType.Elem()).Interface() | |
| } else { | |
| fieldPtrs[i] = field.Addr().Interface() | |
| } | |
| } | |
| } | |
| return fieldPtrs, fieldMap, nil | |
| } | |
| // setStructFields sets the scanned values to the struct fields | |
| func setStructFields(dest any, columns []string, fieldPtrs []interface{}, fieldMap map[string]int) error { | |
| v := reflect.ValueOf(dest).Elem() | |
| for i, column := range columns { | |
| idx, ok := fieldMap[column] | |
| if !ok { | |
| continue | |
| } | |
| field := v.Field(idx) | |
| fieldType := field.Type() | |
| if !field.CanSet() { | |
| return fmt.Errorf("cannot set field %s", column) | |
| } | |
| switch ptr := fieldPtrs[i].(type) { | |
| case *sql.NullTime: | |
| if ptr.Valid { | |
| field.Set(reflect.ValueOf(ptr.Time)) | |
| } else { | |
| field.Set(reflect.ValueOf(time.Time{})) | |
| } | |
| case *sql.NullBool: | |
| if ptr.Valid { | |
| field.SetBool(ptr.Bool) | |
| } else { | |
| field.SetBool(false) | |
| } | |
| case *sql.NullString: | |
| if ptr.Valid { | |
| if fieldType == reflect.TypeOf(uuid.UUID{}) { | |
| u, err := uuid.Parse(ptr.String) | |
| if err != nil { | |
| return fmt.Errorf("error parsing UUID: %w", err) | |
| } | |
| field.Set(reflect.ValueOf(u)) | |
| } else { | |
| field.SetString(ptr.String) | |
| } | |
| } else { | |
| field.Set(reflect.Zero(fieldType)) | |
| } | |
| case *[]byte: | |
| field.SetBytes(*ptr) | |
| default: | |
| if field.Kind() == reflect.Ptr { | |
| if reflect.ValueOf(ptr).Elem().IsValid() { | |
| field.Set(reflect.ValueOf(ptr).Elem()) | |
| } else { | |
| field.Set(reflect.Zero(fieldType)) | |
| } | |
| } else { | |
| field.Set(reflect.ValueOf(ptr).Elem()) | |
| } | |
| } | |
| } | |
| return nil | |
| } | |
| // fetchColumns fetches the columns from a dummy query | |
| func fetchColumns(db *sql.DB, tableName string) ([]string, error) { | |
| query := fmt.Sprintf("SELECT * FROM %s LIMIT 1", tableName) | |
| rows, err := db.Query(query) | |
| if err != nil { | |
| return nil, err | |
| } | |
| defer rows.Close() | |
| return rows.Columns() | |
| } | |
| func cleanTag(tag string) string { | |
| return strings.Split(tag, ",")[0] | |
| } |
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