go - 模型.go : res declared and not used

Question

虽然我更改了数据库名称和密码，但我无法重现此示例。树结构

.
├── main.go
├── main_test.go
├── models
│   └── models.go
└── utils
    └── encodeutils.go

模型

func InitDB() (*sql.DB, error) {
    var err error
    db, err := sql.Open("postgres", "postgres://miki:pass58@localhost/mydb?sslmode=disable")
    if err != nil {
        return nil, err
    } else {
        // Create model for our URL service
        stmt, err := db.Prepare("CREATE TABLE WEB_URL(ID SERIAL PRIMARY KEY, URL TEXT NOT NULL);")
        if err != nil {
            log.Println(err)
            return nil, err
        }
        res, err := stmt.Exec()
        if err != nil {
            log.Println(err)
            return nil, err
        }
        return db, nil
    }
}

主.go

import (
    "database/sql"
    "encoding/json"
    "io/ioutil"
    "log"
    "net/http"
    "time"

    "github.com/PacktPublishing/Building-RESTful-Web-Services-with-Go/Chapter07/urlshortener/models"
    base62 "github.com/PacktPublishing/Building-RESTful-Web-Services-with-Go/Chapter07/urlshortener/utils"
    "github.com/gorilla/mux"
    _ "github.com/lib/pq"
)

// DB stores the database session imformation. Needs to be initialized once
type DBClient struct {
    db *sql.DB
}

// Model the record struct
type Record struct {
    ID  int    `json:"id"`
    URL string `json:"url"`
}

// GetOriginalURL fetches the original URL for the given encoded(short) string
func (driver *DBClient) GetOriginalURL(w http.ResponseWriter, r *http.Request) {
    var url string
    vars := mux.Vars(r)
    // Get ID from base62 string
    id := base62.ToBase10(vars["encoded_string"])
    err := driver.db.QueryRow("SELECT url FROM web_url WHERE id = $1", id).Scan(&url)
    // Handle response details
    if err != nil {
        w.Write([]byte(err.Error()))
    } else {
        w.WriteHeader(http.StatusOK)
        w.Header().Set("Content-Type", "application/json")
        responseMap := map[string]interface{}{"url": url}
        response, _ := json.Marshal(responseMap)
        w.Write(response)
    }
}

// GenerateShortURL adds URL to DB and gives back shortened string
func (driver *DBClient) GenerateShortURL(w http.ResponseWriter, r *http.Request) {
    var id int
    var record Record
    postBody, _ := ioutil.ReadAll(r.Body)
    json.Unmarshal(postBody, &record)
    err := driver.db.QueryRow("INSERT INTO web_url(url) VALUES($1) RETURNING id", record.URL).Scan(&id)
    responseMap := map[string]interface{}{"encoded_string": base62.ToBase62(id)}
    if err != nil {
        w.Write([]byte(err.Error()))
    } else {
        w.Header().Set("Content-Type", "application/json")
        response, _ := json.Marshal(responseMap)
        w.Write(response)
    }
}

func main() {
    db, err := models.InitDB()
    if err != nil {
        panic(err)
    }
    dbclient := &DBClient{db: db}
    if err != nil {
        panic(err)
    }
    defer db.Close()
    // Create a new router
    r := mux.NewRouter()
    // Attach an elegant path with handler
    r.HandleFunc("/v1/short/{encoded_string:[a-zA-Z0-9]*}", dbclient.GetOriginalURL).Methods("GET")
    r.HandleFunc("/v1/short", dbclient.GenerateShortURL).Methods("POST")
    srv := &http.Server{
        Handler: r,
        Addr:    "127.0.0.1:8000",
        // Good practice: enforce timeouts for servers you create!
        WriteTimeout: 15 * time.Second,
        ReadTimeout:  15 * time.Second,
    }
    log.Fatal(srv.ListenAndServe())
}

当我运行时

go run /home/miki/go/src/github.com/PacktPublishing/Building-RESTful-Web-Services-with-Go/Chapter07/urlshortener/main.go

我得到了

go/src/github.com/PacktPublishing/Building-RESTful-Web-Services-with-Go/Chapter07/urlshortener/models/models.go:22:3: res declared and not used

为什么？模型在 main 中初始化。

只是为了展示postqresqlmydb=#\c mydb\dt

You are now connected to database "mydb" as user "postgres".
        List of relations
 Schema |  Name   | Type  | Owner 
--------+---------+-------+-------
 public | web_url | table | miki

如果我尝试

_, err = stmt.Exec()
if err != nil {
    log.Println(err)
    return nil, err
}
return db, nil

然后

2019/04/09 09:35:58 pq: relation "web_url" already exists
panic: pq: relation "web_url" already exists

Answer 1

报错信息清楚，res，model中定义，见下文，已声明未使用

res, err := stmt.Exec()
if err != nil {
  log.Println(err)
  return nil, err
}
return db, nil

如果你不关心res，可以用_忽略它

_, err = stmt.Exec()
if err != nil {
  log.Println(err)
  return nil, err
}
return db, nil