What is the difference between Get String Map UnMarshall Key and UnMarshall

[agileboy365]

package main

import (
"fmt"
"log"
"os"
"strings"

"github.com/spf13/viper"

)

type Config struct {
Root string mapstructure:"root"
App AppConfig mapstructure:"app"
Server ServerConfig mapstructure:"server"
Database DatabaseConfig mapstructure:"database"
}

type AppConfig struct {
Name string mapstructure:"name"
}

type ServerConfig struct {
Port int mapstructure:"port"
}

type DatabaseConfig struct {
Host string mapstructure:"host"
Port int mapstructure:"port"
Name string mapstructure:"name"
MaxOpen int mapstructure:"max_open"
}

func init() {
os.Setenv("MYAPP_ROOT", "Env_Test")
os.Setenv("MYAPP_APP__NAME", "Env_OracleTest")
os.Setenv("MYAPP_SERVER__PORT", "8090")
os.Setenv("MYAPP_DATABASE__HOST", "Env_sulink")
os.Setenv("MYAPP_DATABASE__PORT", "1522")
os.Setenv("MYAPP_DATABASE__NAME", "Env_testclr")
os.Setenv("MYAPP_DATABASE__MAX_OPEN", "30")

viper.AutomaticEnv()
viper.AllowEmptyEnv(true)
viper.SetEnvPrefix("MYAPP")
viper.SetEnvKeyReplacer(strings.NewReplacer(".", "__"))
bindEnvVars()

viper.SetConfigName("config")
viper.AddConfigPath(".")
viper.SetConfigType("yaml")

if err := viper.ReadInConfig(); err != nil {
	log.Printf("Error reading config file, %s. Using environment variables only.", err)
}

fmt.Printf("AllSettings:%+v\n", viper.GetViper().AllSettings())

var cfg Config
if err := viper.Unmarshal(&cfg); err != nil {
	log.Fatalf("Unable to decode into struct, %v\n", err)
	return
}
fmt.Printf("Config loaded: %+v\n", cfg)

fmt.Printf("DatabaseMap: %+v\n", viper.GetStringMap("database"))
var dbConfig DatabaseConfig
if err := viper.UnmarshalKey("database", &dbConfig); err != nil {
	log.Fatalf("Unable to decode database config into struct, %v", err)
}
fmt.Printf("DatabaseConfig loaded: %+v\n", dbConfig)

}

func bindEnvVars() {
viper.BindEnv("root", "MYAPP_ROOT")
viper.BindEnv("app.name", "MYAPP_APP__NAME")
viper.BindEnv("server.port", "MYAPP_SERVER__PORT")
viper.BindEnv("database.host", "MYAPP_DATABASE__HOST")
viper.BindEnv("database.port", "MYAPP_DATABASE__PORT")
viper.BindEnv("database.name", "MYAPP_DATABASE__NAME")
viper.BindEnv("database.max_open", "MYAPP_DATABASE__MAX_OPEN")
}

func main() {
root := viper.GetString("root")
appName := viper.GetString("app.name")
serverPort := viper.GetInt("server.port")
dbHost := viper.GetString("database.host")
maxOpen := viper.GetInt("database.max_open")

fmt.Println("Root:", root)
fmt.Println("Application Name:", appName)
fmt.Println("Server Port:", serverPort)
fmt.Println("Database Host:", dbHost)
fmt.Println("Database MaxOpen:", maxOpen)

}

Output
Starting: C:\Users\39636\go\bin\dlv.exe dap --listen=127.0.0.1:62021 from d:\Study\Go\oracletest
2025-03-19T18:16:51+08:00 warn layer=dlv CGO_CFLAGS already set, Cgo code could be optimized.
DAP server listening at: 127.0.0.1:62021
Type 'dlv help' for list of commands.
AllSettings:map[app:map[name:Env_OracleTest] database:map[host:Env_sulink max_open:30 name:Env_testclr port:1522] root:Env_Test server:map[port:8090]]
Config loaded: {Root:Env_Test App:{Name:Env_OracleTest} Server:{Port:8090} Database:{Host:Env_sulink Port:1522 Name:Env_testclr MaxOpen:30}}
DatabaseMap: map[host:sulink max_open:10 name:testclr port:1521]
DatabaseConfig loaded: {Host:sulink Port:1521 Name:testclr MaxOpen:10}
Root: Env_Test
Application Name: Env_OracleTest
Server Port: 8090
Database Host: Env_sulink
Database MaxOpen: 30
Process 20056 has exited with status 0

Viper.getStringMap ("database"), viper.UnmarshalKey ("database",&dbConfig) with the value of config. yaml
The value of viper. Unimarshal (&cfg) is the environment variable
My true intention is to prioritize environment variables, regardless of whether this KEY exists in the configuration file or not