I am currently working on a feature where I will be able to see all the tweets from the users I follow, i am testing it using Postman, but I cannot figure out what the issue is, I know it has to do with a nil or pointer deference, but I haven't been able to figure it out despite the careful readings I have done on my code. I am pretty much new to Go and this is the first language I learn that uses this sort of syntax and that uses pointers so I apologize in advanced if it is something obvious I am not seeing. On my db package
package db
import (
"context"
"time"
"fmt"
"github.com/JeffersonGarcia15/Twitter-Clone/models"
"go.mongodb.org/mongo-driver/bson"
)
/*
ReadFollowersTweets reads the tweets from my followers
*/
func ReadFollowersTweets(ID string, page int) ([]models.ReturnFollowersTweets, bool) {
ctx, cancel := context.WithTimeout(context.Background(), 15*time.Second)
defer cancel()
db := MongoCN.Database("twitter")
col := db.Collection("joins")
skip := (page - 1) * 20
conditions := make([]bson.M, 0)
conditions = append(conditions, bson.M{"$match": bson.M{"userid": ID}})
conditions = append(conditions, bson.M{
"$lookup": bson.M{
"from": "tweets",
"localField": "userrelationid",
"foreignField": "userid",
"as": "tweets",
}})
conditions = append(conditions, bson.M{"unwind": "$tweets"}) //allows all the info to be repeated with the same structure for all tweets on same page
conditions = append(conditions, bson.M{"$sort": bson.M{"date": "desc"}})
conditions = append(conditions, bson.M{"$skip": skip})
conditions = append(conditions, bson.M{"$limit": 20})
cursor, err := col.Aggregate(ctx, conditions)
var result []models.ReturnFollowersTweets
err := cursor.All(ctx, &result)
if err != nil {
fmt.Println(err.Error())
return result, false
}
return result, true
}
On routers
package routers
import (
"encoding/json"
"net/http"
"strconv"
"github.com/JeffersonGarcia15/Twitter-Clone/db"
)
/*
ReadFollowersTweets reads the tweets of all our followers
*/
func ReadFollowersTweets(w http.ResponseWriter, r *http.Request) {
if len(r.URL.Query().Get("page")) < 1 {
http.Error(w, "You must send a page number", http.StatusBadRequest)
return
}
page, err := strconv.Atoi(r.URL.Query().Get("page"))
if err != nil {
http.Error(w, "You must send a page number as an int greater than 0" err.Error(), http.StatusBadRequest)
return
}
response, correct := db.ReadFollowersTweets(IDUser, page)
if !correct {
http.Error(w, "An error occurred when reading the tweets", http.StatusBadRequest)
return
}
w.Header().Set("Content-Type", "application/json")
w.WriteHeader(http.StatusCreated)
json.NewEncoder(w).Encode(response)
}
Error in console
2021/10/04 10:24:46 http: panic serving [::1]:55085: runtime error: invalid memory address or nil pointer dereference
goroutine 164 [running]:
net/http.(*conn).serve.func1()
/usr/local/Cellar/go/1.17/libexec/src/net/http/server.go:1801 0xb9
panic({0x14bbac0, 0x1985c40})
/usr/local/Cellar/go/1.17/libexec/src/runtime/panic.go:1047 0x266
go.mongodb.org/mongo-driver/mongo.(*Cursor).closeImplicitSession(0x10526ff)
/Users/jeffersonlopezgarcia/go/pkg/mod/go.mongodb.org/mongo-driver@v1.7.2/mongo/cursor.go:267 0x14
panic({0x14bbac0, 0x1985c40})
/usr/local/Cellar/go/1.17/libexec/src/runtime/panic.go:1038 0x215
go.mongodb.org/mongo-driver/mongo.(*Cursor).Close(0x0, {0x16755a8, 0xc000416360})
/Users/jeffersonlopezgarcia/go/pkg/mod/go.mongodb.org/mongo-driver@v1.7.2/mongo/cursor.go:180 0x5f
panic({0x14bbac0, 0x1985c40})
/usr/local/Cellar/go/1.17/libexec/src/runtime/panic.go:1038 0x215
go.mongodb.org/mongo-driver/mongo.(*Cursor).All(0x0, {0x16755a8, 0xc000416360}, {0x1487380, 0xc00000e600})
/Users/jeffersonlopezgarcia/go/pkg/mod/go.mongodb.org/mongo-driver@v1.7.2/mongo/cursor.go:209 0x1fe
github.com/JeffersonGarcia15/Twitter-Clone/db.ReadFollowersTweets({0xc0000fc0c0, 0x18}, 0x1)
/Users/jeffersonlopezgarcia/go/src/github.com/JeffersonGarcia15/Twitter-Clone/db/readFollowersTweets.go:41 0x8c9
github.com/JeffersonGarcia15/Twitter-Clone/routers.ReadFollowersTweets({0x1673cd0, 0xc000588700}, 0xc000684300)
/Users/jeffersonlopezgarcia/go/src/github.com/JeffersonGarcia15/Twitter-Clone/routers/readFollowersTweets.go:24 0x10a
net/http.HandlerFunc.ServeHTTP(...)
/usr/local/Cellar/go/1.17/libexec/src/net/http/server.go:2046
github.com/JeffersonGarcia15/Twitter-Clone/middlew.ValidJWT.func1({0x1673cd0, 0xc000588700}, 0xc000684300)
/Users/jeffersonlopezgarcia/go/src/github.com/JeffersonGarcia15/Twitter-Clone/middlew/validJWT.go:20 0xb2
net/http.HandlerFunc.ServeHTTP(...)
/usr/local/Cellar/go/1.17/libexec/src/net/http/server.go:2046
github.com/JeffersonGarcia15/Twitter-Clone/middlew.CheckDB.func1({0x1673cd0, 0xc000588700}, 0xc00059a600)
/Users/jeffersonlopezgarcia/go/src/github.com/JeffersonGarcia15/Twitter-Clone/middlew/checkDB.go:18 0xa9
net/http.HandlerFunc.ServeHTTP(0xc000684200, {0x1673cd0, 0xc000588700}, 0xc0000a2701)
/usr/local/Cellar/go/1.17/libexec/src/net/http/server.go:2046 0x2f
github.com/gorilla/mux.(*Router).ServeHTTP(0xc0004ba240, {0x1673cd0, 0xc000588700}, 0xc000684100)
/Users/jeffersonlopezgarcia/go/pkg/mod/github.com/gorilla/mux@v1.8.0/mux.go:210 0x1cf
github.com/rs/cors.(*Cors).Handler.func1({0x1673cd0, 0xc000588700}, 0xc000684100)
/Users/jeffersonlopezgarcia/go/pkg/mod/github.com/rs/cors@v1.8.0/cors.go:219 0x1bd
net/http.HandlerFunc.ServeHTTP(0xc000559819, {0x1673cd0, 0xc000588700}, 0x106236e)
/usr/local/Cellar/go/1.17/libexec/src/net/http/server.go:2046 0x2f
net/http.serverHandler.ServeHTTP({0xc00059a450}, {0x1673cd0, 0xc000588700}, 0xc000684100)
/usr/local/Cellar/go/1.17/libexec/src/net/http/server.go:2878 0x43b
net/http.(*conn).serve(0xc00031d4a0, {0x16755e0, 0xc00059a2d0})
/usr/local/Cellar/go/1.17/libexec/src/net/http/server.go:1929 0xb08
created by net/http.(*Server).Serve
/usr/local/Cellar/go/1.17/libexec/src/net/http/server.go:3033 0x4e8
CodePudding user response：
The last line from the stack trace that's in your code is:
/Users/jeffersonlopezgarcia/go/src/github.com/JeffersonGarcia15/Twitter-Clone/db/readFollowersTweets.go:41
That appears to be, from the quoted code:
err := cursor.All(ctx, &result)
Let's see, how could that cause nil pointer? Let's see where the relevant parts are initialized.
cursor, _ := col.Aggregate(ctx, conditions)
You're discarding an error here, so it seems pretty likely this is your cause. Never discard your errors, especially when something is failing and you can't figure out why. That should always be the first thing you look for.