Revert "Revert "Fix Viper 'UnmarshalKey' behavior to include all sources

viper.go

@@ -865,13 +865,27 @@ func UnmarshalKey(key string, rawVal interface{}, opts ...DecoderConfigOption) e
 	return v.UnmarshalKey(key, rawVal, opts...)
 }
 func (v *Viper) UnmarshalKey(key string, rawVal interface{}, opts ...DecoderConfigOption) error {
-	err := decode(v.Get(key), defaultDecoderConfig(rawVal, opts...))
+	lcaseKey := strings.ToLower(key)
 
-	if err != nil {
-		return err
-	}
+	// AllSettings returns settings from every sources merged into one tree
+	settings := v.AllSettings()
 
-	return nil
+	keyParts := strings.Split(lcaseKey, v.keyDelim)
+	for i := 0; i < len(keyParts)-1; i++ {
+		if value, found := settings[keyParts[i]]; found {
+			if valueMap, ok := value.(map[string]interface{}); ok {
+				settings = valueMap
+				continue
+			}
+			// if the current value is not a map[string]interface{} we most likely reach a
+			// leaf and the key/path is wrong
+			return fmt.Errorf("unknown key %s", lcaseKey)
+		} else {
+			return fmt.Errorf("unknown key %s", lcaseKey)
+		}
+	}
+	finalSetting := settings[keyParts[len(keyParts)-1]]
+	return decode(finalSetting, defaultDecoderConfig(rawVal, opts...))
 }
 
 // Unmarshal unmarshals the config into a Struct. Make sure that the tags

viper_test.go

@@ -1725,6 +1725,48 @@ func TestParseNested(t *testing.T) {
 	assert.Equal(t, 200*time.Millisecond, items[0].Nested.Delay)
 }
 
+func TestUnmarshalKey(t *testing.T) {
+	type testStruct struct {
+		Delay int      `mapstructure:"delay" yaml:"delay"`
+		Port  int      `mapstructure:"port" yaml:"port"`
+		Host  string   `mapstructure:"host" yaml:"host"`
+		Items []string `mapstructure:"items" yaml:"items"`
+	}
+
+	config := `
+level_1:
+  level_2:
+    port: 1234
+    items:
+      - "test 1"
+`
+	v := New()
+	v.SetDefault("level_1.level_2.delay", 50)
+	v.SetDefault("level_1.level_2.port", 9999)
+	v.SetDefault("level_1.level_2.host", "default")
+	v.SetDefault("level_1.level_2.items", []string{})
+
+	// Use env vars for some settings
+	v.BindEnv("level_1.level_2.host", "DD_TEST_HOST")
+	t.Setenv("DD_TEST_HOST", "dd.com")
+
+	initConfig(v, "yaml", config)
+
+	// manually overwrite some settings
+	v.Set("level_1.level_2.items", []string{"test_2", "test_3"})
+
+	data := testStruct{}
+	err := v.UnmarshalKey("level_1.level_2", &data)
+	if err != nil {
+		t.Fatalf("unable to decode into struct, %v", err)
+	}
+
+	assert.Equal(t, 50, data.Delay)                           // value from defaults
+	assert.Equal(t, 1234, data.Port)                          // value from config
+	assert.Equal(t, "dd.com", data.Host)                      // value from env
+	assert.Equal(t, []string{"test_2", "test_3"}, data.Items) // value from Set()
+}
+
 func doTestCaseInsensitive(t *testing.T, typ, config string) {
 	v := New()
 	initConfig(v, typ, config)