package main

import (
   "context"
   "fmt"
   "log"
   "time"

   "go.mongodb.org/mongo-driver/mongo"
   "go.mongodb.org/mongo-driver/mongo/options"
)

type User struct {
   ID        string
   Name      string
   Followers map[string]*User
   Friends   map[string]*User
}

func main() {

   ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
   defer cancel()

   client, err := mongo.Connect(ctx, options.Client().ApplyURI("mongodb://localhost:27017"))

   err = client.Ping(ctx, nil)
   if err != nil {
      log.Fatal("could not connect")
   }

   coll := client.Database("testing").Collection("users")

   ctx, cancel = context.WithTimeout(context.Background(), 5*time.Second)
   defer cancel()

   u1 := &User{ID: "24", Name: "Test1", Followers: map[string]*User{}, Friends: map[string]*User{}}
   u2 := &User{ID: "25", Name: "Test2", Followers: map[string]*User{}, Friends: map[string]*User{}}

   u1.Friends[u2.ID] = u2
   u2.Followers[u1.ID] = u1
   
   // uncommenting the next line and commenting out the two above gives the same behavior i.e. assigning to self
   //u1.Followers[u1.ID] = u1 

   res, err := coll.InsertOne(ctx, u1)
   if err != nil {
      fmt.Println("error:", err)
   }
   fmt.Println(res.InsertedID)
}

runtime: goroutine stack exceeds 1000000000-byte limitruntime: goroutine stack exceeds 1000000000-byte limitfatal error: stack overflow