From 6d96b16143c37dc3b735068c55154121d09921a6 Mon Sep 17 00:00:00 2001
From: Kolbe Kegel <kolbe>
Date: Tue, 22 Aug 2017 23:35:11 -0700
Subject: [PATCH 1/2] Added UnmarshalKeyExact() as well as non-method
 UnmarshalExact().

---
 viper.go | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/viper.go b/viper.go
index 2a221e5..4b93427 100644
--- a/viper.go
+++ b/viper.go
@@ -768,6 +768,7 @@ func decode(input interface{}, config *mapstructure.DecoderConfig) error {
 
 // UnmarshalExact unmarshals the config into a Struct, erroring if a field is nonexistent
 // in the destination struct.
+func UnmarshalExact(rawVal interface{}) error { return v.UnmarshalExact(rawVal) }
 func (v *Viper) UnmarshalExact(rawVal interface{}) error {
 	config := defaultDecoderConfig(rawVal)
 	config.ErrorUnused = true
@@ -782,6 +783,23 @@ func (v *Viper) UnmarshalExact(rawVal interface{}) error {
 
 	return nil
 }
+// UnmarshalKeyExact takes a single key and unmarshals it into a Struct, erroring if a field
+// is nonexistent in the destination struct.
+func UnmarshalKeyExact(key string, rawVal interface{}) error { return v.UnmarshalKeyExact(key, rawVal) }
+func (v *Viper) UnmarshalKeyExact(key string, rawVal interface{}) error {
+	config := defaultDecoderConfig(rawVal)
+	config.ErrorUnused = true
+
+	err := decode(v.Get(key), config)
+
+	if err != nil {
+		return err
+	}
+
+	v.insensitiviseMaps()
+
+	return nil
+}
 
 // BindPFlags binds a full flag set to the configuration, using each flag's long
 // name as the config key.

From eb4fbe638fa59a5a9fff50a3ff0f7f023c720b04 Mon Sep 17 00:00:00 2001
From: Kolbe Kegel <kolbe>
Date: Tue, 22 Aug 2017 23:48:57 -0700
Subject: [PATCH 2/2] Added TestUnmarshalKeyExact() to test
 UnmarshalKeyExact().

---
 viper_test.go | 31 +++++++++++++++++++++++++++++++
 1 file changed, 31 insertions(+)

diff --git a/viper_test.go b/viper_test.go
index 774ca11..7cd832c 100644
--- a/viper_test.go
+++ b/viper_test.go
@@ -287,6 +287,37 @@ func TestUnmarshalExact(t *testing.T) {
 	}
 }
 
+func TestUnmarshalKeyExact(t *testing.T) {
+	type duration struct {
+		Delay time.Duration
+	}
+
+	type item struct {
+		Name   string
+		Delay  time.Duration
+		Nested duration
+	}
+
+	config := `invalid_but_irrelevant=true
+        [[parent]]
+        error=true
+	delay="100ms"
+	[parent.nested]
+	delay="200ms"
+`
+	initConfig("toml", config)
+
+	var items []item
+	err := v.UnmarshalKeyExact("parent", &items)
+	if err == nil {
+		t.Fatal("UnmarshalKeyExact should error when populating a struct from a conf that contains unused fields")
+	}
+
+	assert.Equal(t, 1, len(items))
+	assert.Equal(t, 100*time.Millisecond, items[0].Delay)
+	assert.Equal(t, 200*time.Millisecond, items[0].Nested.Delay)
+}
+
 func TestOverrides(t *testing.T) {
 	Set("age", 40)
 	assert.Equal(t, 40, Get("age"))