add tutorial for the official mongodb go driver

Former-commit-id: 8353dd101c37c223bba404403f9f8fa2d042fede

_examples/tutorial/mongodb/env/env.go

@@ -0,0 +1,71 @@
+package env
+
+import (
+	"fmt"
+	"log"
+	"os"
+	"path/filepath"
+	"strings"
+
+	"github.com/joho/godotenv"
+)
+
+var (
+	// Port is the PORT environment variable or 8080 if missing.
+	// Used to open the tcp listener for our web server.
+	Port string
+	// DSN is the DSN environment variable or mongodb://localhost:27017 if missing.
+	// Used to connect to the mongodb.
+	DSN string
+)
+
+func parse() {
+	Port = getDefault("PORT", "8080")
+	DSN = getDefault("DSN", "mongodb://localhost:27017")
+}
+
+// Load loads environment variables that are being used across the whole app.
+// Loading from file(s), i.e .env or dev.env
+//
+// Example of a 'dev.env':
+// PORT=8080
+// DSN=mongodb://localhost:27017
+//
+// After `Load` the callers can get an environment variable via `os.Getenv`.
+func Load(envFileName string) {
+	if args := os.Args; len(args) > 1 && args[1] == "help" {
+		fmt.Fprintln(os.Stderr, "https://github.com/kataras/iris/blob/master/_examples/tutorials/mongodb/README.md")
+		os.Exit(-1)
+	}
+
+	log.Printf("Loading environment variables from file: %s\n", envFileName)
+	// If more than one filename passed with comma separated then load from all
+	// of these, a env file can be a partial too.
+	envFiles := strings.Split(envFileName, ",")
+	for i := range envFiles {
+		if filepath.Ext(envFiles[i]) == "" {
+			envFiles[i] += ".env"
+		}
+	}
+
+	if err := godotenv.Load(envFiles...); err != nil {
+		panic(fmt.Sprintf("error loading environment variables from [%s]: %v", envFileName, err))
+	}
+
+	envMap, _ := godotenv.Read(envFiles...)
+	for k, v := range envMap {
+		log.Printf("◽ %s=%s\n", k, v)
+	}
+
+	parse()
+}
+
+func getDefault(key string, def string) string {
+	value := os.Getenv(key)
+	if value == "" {
+		os.Setenv(key, def)
+		value = def
+	}
+
+	return value
+}